Still sleeping with and seeing my ex boyfriend regularly even though he’s 4/5 months into a happy, new relationship. Please convince me that it’s wrong.

7 months ago I (F 22) ended my 6 year relationship with my ex (24) which got extremely toxic especially towards the end. He spent 2 months doing everything he possibly could to get me back but I refused each time despite it being gut wrenching saying no to him every time, but I knew what was best for me and our relationship wasn’t that. We didn’t speak for a month in December, which later I found out was because he got into a new relationship with let’s call her Sarah. I was doing well(ish), although still feeling somewhat heartbroken and down. In January , he broke contact and we saw eachother a few weeks later. Since then, we have been sleeping together and seeing eachother. We’ve had sex countless times since then, sometimes 3 or more times a week. At the start of February, I found out he had a girlfriend and was also seeing other women, as well as me, and cut contact. However, he came back into my life a few weeks later and we started again. I want to express that these last few months have been the worst months of my entire life, I have been completely heartbroken and hurt more than I ever have. My life is in shambles, I’m depressed, extremely anxious and my values and morals have gone out the window (clearly). I tried many, many times to cut contact but he always ends up back in my life. I did it again in march, because I found out that he was now only seeing Sarah , and that essentially she wasn’t just a girl he was using to get over me, but someone he sees a future with her and apparently, is in love with. (But not enough to leave me alone). He also tells me she is very much in love with him , and very vaguely mentioned something about mental health issues and her being depressed until she met him. I’m still so, so in love with him and I just don’t know what to do. I think part of the reason I keep agreeing to see him and sleep with him is because as long as he’s with me, he’s not with her. That sounds awful but it’s true. Now for the worse part - I don’t feel guilty, not even an ounce of guilt, or feeling bad towards Sarah. I’ll even go as far as to feel content after sleeping with him, even if she calls during or after, because it feels like I have the upper hand. I’m angry at her for being with him, even though she doesn’t know me and didn’t do anything wrong - but it feels as though she has what’s mine. I can acknowledge this is objectively wrong, but I don’t feel it. This sounds despicable as I’m typing, but I’m being 100% honest . I have tried everything to properly get over him and stop seeing him and nothing has worked so I need a new perspective. If i come out of this weird delusional state im hoping i can have a fresh perspective on the situation and regain my lost morals, because this isn’t who i am. I’m welcoming anything really, stories about how it feels to be Sarah in this situation, tough love, whatever. I need a reality check.

TL; DR: although I know it’s objectively wrong, I don’t feel bad or guilty for sleeping with my ex of 6 years who is now in a new 5 month relationship. Please help me understand why I should feel guilty