Ex fiancé/boyfriend

I will start by saying i’m a lurker. lol

a little backstory, my ex fiancé, now boyfriend got together in May 2015. He left me in July 2018. the day he got all of his stuff out of my house (and fucked me) he moved in with an ex he’s been on and off with since 2008 and openly admitted to sleeping with her the same night. He began working the same job as her delivering pizza. he went into almost 4,000 debt in her name over financed car parts. In Sept this year he reached back out to me, we spent a night in a hotel together, and he went back home to her for a month (to apparently keep using her for car parts) then on october 23 we decided to get back together and he moved back in with me.

the problem i am struggling with; he and her both work nights at a pizza place that closes at 3am. they are lucky to get out by 5am. part of me worries (insecurities, i know) that he is going to cheat on me with her because i’m in bed at that time and i would never know. she lives 5 mins away from me. he has done nothing but be good to me since we have been back together, all he has done is assure me that he loves me and not her, and that if he wanted her he wouldn’t have left her. all i do is fuss over what he has done to me, all i feel is hurt because i live in the past. i know i need to move forward, because i wasn’t innocent while we were split up either, but at the same time it was him that left me, he left the door open for anything. my heart hurts, so bad still. i live on repeat every single day, fussing and yelling about how could he do this to me, when i never could’ve done it to him. i don’t know how to move past all the hurt, i just want to leave the past behind and look at the future.

i guess my question is; is there anyone that can give advice on how to cope? how to heal and just look forward? he is so good to me, i just want to be happy and stop feeling so much pain.