My (19F) boyfriend (19M) broke my trust and i don't know how to trust him again.

We've been together for the past year. Our relationship is currently in a very delicate place because, before me, he had already been with many other girls. I understand that because I’ve been in previous relationships too, but the difference is that before entering a new relationship, I always make sure I’m 1000% over what happened between me and my ex-partner. I never carry anything that reminds me of them. I delete every single thing related to them on social media, unfollow or block them everywhere, because I personally don’t believe in being in contact or friends with an ex while being in a new relationship. It depends on the situation, but most of the time it’s a no for me, especially if I never even got to meet the person before me. I don’t see a reason to keep them around at all, especially when the previous relationship was toxic in any way. And that’s just my opinion, I’m not claiming I’m right, but that’s how I wish to be respected in a relationship.
The problem is that my boyfriend is kind of neutral regarding this.
My boyfriend was in a relationship with his ex for about two and a half years, and about six months later we started seeing each other. I couldn’t tell for sure if he was over her already, but I could feel something was off.
There were several things involving his ex that deeply affected me. He still had a picture of both of them together along with other family members hanging near the bed. I told him it made me uncomfortable to the point it was unbearable. He reluctantly took it off the wall, claiming that his grandfather who passed away was still in the picture. I explained that he could still keep the picture, just not literally right next to where we sleep, especially because I know he has many other pictures with his family that don’t include her. It made me feel sick thinking she got to meet his grandfather and I never did, although that’s not even the main point.
His Twitter is also still filled with old things from when they were together. I understand deleting things one by one is difficult, but emotionally it’s still unbearable for me to constantly see traces of that relationship everywhere.
His friend once joked about his ex in front of me, and I cried for hours because this whole situation became such a huge emotional trigger for me.
I usually don’t suffer from retroactive jealousy, but with this situation it hurts so badly because I truly feel like he wasn’t completely over her when we started seeing each other.
At the same time, between his ex and me, he was also seeing another girl that he had nothing serious with. They went out a few times, and he told her he wasn’t interested in anything serious, although she still wanted something more with him. Eventually they stopped seeing each other.
The problem is that they continued talking “as friends” well into our relationship, months after we became serious, after we fully entered the boyfriend-and-girlfriend stage, after I met all his family and friends, and after we were basically spending all our time together exclusively.
I had always suspected something was wrong, and honestly I only did what I did because I genuinely felt it. In January of this year, I went through his phone and searched for their conversation. The conversation only stopped because she eventually stopped replying to him around the same time she entered a new relationship herself.
What I found made me feel sick. There was nothing explicitly proving cheating, but the way he talked to her felt completely wrong to me. He was overly friendly, saying things like “I’ll get you one of these someday” regarding a Pokémon card she wanted.
She also spammed him with reels constantly, and I knew that because I would see her notifications whenever I was with him.
What hurts me the most is that he hid all of this from me the entire time. I repeatedly asked him if he still had contact with anyone from his past? an ex or someone he had been seeing before me, and he always said no. That was a lie.
When I confronted him, he claimed he simply forgot about it and that it meant nothing to him, but mentally this has destroyed me for months.
Yesterday we talked about it again, and I kept asking him questions I already knew the answers to, like whether they had been intimate. He denied it, but I already knew the truth because I had seen a picture of him shirtless in her bed from the time they were still seeing each other.
It hurts so much that he hid these things from me.
I’m diagnosed with borderline personality disorder, but I’ve genuinely tried my best not to become the person destroying the relationship. Still, now I feel completely sick emotionally. I feel obsessed with these girls. I constantly compare myself to them and feel like I’m not enough. I feel like my trust has been betrayed, and now it feels almost impossible to fully trust him again, even with simple things like him going out alone at night.
At the same time, I can’t fully bring myself to admit I don’t trust him because I love him so much. It hurts to even think about ending the relationship, and part of me feels selfish for struggling this much when technically he “could have done something worse.”
He tells me I’m not overreacting and says he feels sorry, but at the same time he still avoids being completely transparent and only really apologizes when I push him enough for him to admit guilt. That makes it hard for me to feel reassured.
I also feel guilty myself. I feel like somehow this is my fault, even though at the same time I feel deeply betrayed. It feels unfair because all of this emotional damage landed on me, and now I’m the one expected to somehow process it, move on from it, and heal from it.
Physically and emotionally, this situation has destroyed me. I can barely sleep anymore. I lost weight, I’m constantly tense, I have migraines all the time, and I feel like crying every few hours. Most nights I can’t rest without overthinking everything until I cry.
He says he’s willing to do anything necessary to improve the relationship and help me trust him again, but honestly I don’t even know what I need from him anymore. I don’t know what to ask for because nothing feels sufficient.
I’m currently trying to contact a psychiatrist and waiting for a therapist through my health insurance, because I know I need professional help too. But even with therapy, I still feel lost trying to figure out what both of us could possibly do to finally move past this.
People always say communication is the solution, but we’ve already talked about this endlessly and it still hasn’t changed how I feel. I told him I need constant reassurance, and he says he already gives me reassurance, but I still feel like something important is missing. I just genuinely can’t identify what it is.
Another thing that deeply affects me is that I basically had to beg him to officially acknowledge me as his girlfriend and make the relationship official. At one point he even tried breaking up with me because he “had a lot going on,” and now I constantly live with the fear that he could leave me over every small mistake I make.
I’m also usually the one suggesting dates, asking to post each other, or trying to make the relationship feel more publicly affectionate. Even though he does show me affection, he’s nowhere near as “loud” about loving me as he apparently was with previous partners. Maybe he changed as a person, but part of me still wishes he naturally treated me that way too.
He has changed some things after our conversations, but it still doesn’t feel natural to me. I don’t know what else to do anymore, and this hurts so much that I genuinely feel like I can’t take it anymore. I really need help.