Search Results: on and off relationship advice

My best friend who's a little bit younger than me is in her first relationship, 3 months so far. Things have been pretty awful in it. She's got quite a lot of issues and is living alone at 17, I guess you could say she's a little high maintenance. The boy she is with, 21(not a fan of the age gap either, but it's legal where I live), is quite normal and lives a very everything's handed to him life. He comes down from another state to see her and they met online.
As soon as they started dating it was a train wreck, he was continually picking on her about her mental health problems and doing things to purposefully scare her (switching lights on and off in the pitch black house till the point she cried). İmmediately I said 'f*ck him off' and our other friend and I spoke to him about it because she was scared to. At this point she liked him, he liked her, so she wanted to keep trying in hopes maybe he just didn't understand.
Fast forward to now, the bullying stopped but they argue daily and he yells at her almost everyday he is here about something ridiculous (eg. her asking if he left knife marks on the kitchen bench. He lies saying he didn't then later admits he did and she asks why. Then he starts yelling at her) she's tried out acting call everytime it happens to make sure it wasn't her, and he will still yell at her. He can't take jokes but he expects her to take all of his mean bullying. She's on a dating site right now, which he is aware of, and believes that she could for sure find better.
She's unhappy, feels as if she hates him, they have nothing in common, it's a huge train wreck. I've mostly convinced her to separate with him because it's affecting her, he goes back home in two days and she wanted to break up with him now but she's scared so she might do it when he leaves. she's scared she will regret it, be lonely, and he will either be upset or not care at all and keeps second guessing herself. Does anyone have any advice I could pass onto her? Boys and girls!

So, some context. My boyfriend and I were together for almost 3 years. We moved to another state together, and we had been living together for the last year. There were a lot of things coming in between our relationship, like how we were living with my mom (awk, I know, things were complicated and we couldn't afford to live on our own yet), we didn't have the same days off so we hardly saw each other, etc. It got to the point where he only wanted me around for sex in exchange for him doing things for me like cooking dinner or driving me to work which was down the street. It didn't feel like we were a couple anymore.

About a month ago, my coworker was worried about my well-being and expressed how he always thought I was pretty and how he wanted the best for me, and it was nice hearing that from somebody at the time. I ended up inviting him over to the house while my boyfriend was at work and we did sexual things. My boyfriend ended up finding out what happened after looking through my phone and I was trying to explain why I felt the need to do that, but he wasn't believing me. I broke his trust. I owned up to what I did but he was too hurt and has been hurting ever since and it kills me. I've started therapy and got an STD test which came back negative, and I'm working on myself to become a better person, but I feel like it's too late since he already closed his heart off to me.

TLDR: I cheated with my boyfriend of 3 years and he doesn't trust me and doesn't want to take me back. I'm having trouble moving on because he was my everything.

What should I do? Should I keep trying to fix our relationship? How can I prove to him that I've changed and I won't ever do what I did to him again after going through the pain of losing him? How can I get him to trust me again? If you've been cheated on, did you work things out with your partner? I still feel like a shitty human being, but I'm trying to fix what I did wrong.. any help or advice would be appreciated. :(