ex made a promise and I want to trust him

My partner of 4 years originally broke up with me two weeks ago, it came out of NOWHERE and he blamed everything and anything on me, said incredibly awful and rude things, packed up all his things and moved out the same day. We loved each other so incredibly much, lived together, had a dog together, shared all of our friends etc. We went no contact immediately, mostly because I was so so angry but also because he was so cold he probably wouldn’t have responded if I reached out anyway. About a week into it, he texts me and tells me he’d like to come get the rest of his things and talk with me as well.

Fast forward to saturday, he shows up and we go someplace outside the house (roommates) to talk. We sit down and he’s being cold, very matter of fact, and just not showing any emotion whatsoever. However, everything he was saying was very emotionally mature. Apologizing for everything, taking accountability, even telling me that all the reasons and horrible things he told me when we first broke up were mostly projection. He said he’d been doing a lot of self reflection and didn’t want me to feel like it was my fault or even that the reason for the breakup was me. He told me that as much as he (still) loves me that he needs to be alone and fix “the monster inside of him” and that if he “doesn’t do anything about this it will ruin anything he tries to do in life.”

Obviously I was SOBBING at this point, and eventually he started to show emotion, started crying too (in the four years of dating i’d only ever seen him cry 3 times) but he was like actually bawling. The conversation then got very emotional overall, I even suggested long distance to try and get him to stay because I was desperate as that point he is the love of my life. He told me no, and that he HAS to be alone to fix this. I asked him if he’d ever come back, he told me that when he’s ready he promises that I will be the first person he calls. I asked if he planned to see other people, he told me that he doesn’t because he loves me and also the whole needing to be alone thing. But he also made me promise that I would not wait for him, that I would go and live my life and make it happy and fulfilling and surround myself with people who respect me and treat me well. I reiterated his before statement about calling me when he’s ready, and he swore to me that he is going to call me when he feels ready, swore on our dog and his whole family that he will reach out to me again one day. Before he left we kissed (we actually kissed a lot during, this talk was about 12 hours long), he told me he loved me, and that this wasn’t goodbye, just see you again soon.

I know that I obviously cannot hold onto hope too strongly simply for the fact that there’s no real timeline here, and I would just be torturing myself, but based on his words and actions, should I believe that one day I will hear from him again and we can get to start over, or should I prepare myself to see him with another girl in 2 months. Anybody else have a similar experience to share either? I’m just incredibly anxious and I don’t want to break no contact just to be like “were you serious? did you mean it?” If anybody has ANY input, please do share.

TL;DR My ex told me he stills loved me but that he needs to do some serious work on his mental health, but promised i’d be the first person he calls when he feels ready, what do i believe?