I cheated and I’m messed up.

I’ve tried everything, counselling, google, everything I can do reddit is my last option. 
Before I continue, I know I’m a fucked up person. I can already imagine what the comments are gonna be here. 
About 5 months ago, I cheated on my girlfriend of nearly two years. I’m so insanely in love with her, she has helped me so much, she’s got a heart of gold. 
This was the first time I had ever cheated. I have bpd which makes it really hard for me to get over people and things. I cheated with my best friend at the time. 
I decided to cut of all contact with the best friend ( as I should have). 
I feel ashamed to say it but it’s been so hard for me. It’s so hard to let people go, I care for people too much. I miss my best friend, just as my best friend. It’s so overwhelming right now. My first priority will always be my girlfriend and I never want to lose her but I’m struggling so much. 
I forgive people way too easily for things they’ve done to me hence why it takes me years to get over every ex of mine but I’m at a loss for what to do. Sometimes I catch myself thinking about my best friend and the sex we could have and I feel sick. I get these intrusive thoughts and I’m disgusted in myself that sometimes it pleases me. 
I just want my best friend back and to keep my girlfriend but I know I’m crazy for thinking that could ever work which I know it shouldn’t even be a possibility. It also means I shouldn’t speak to my best friend because then I’ll miss her more but fuck, this hurts so much. 
Roast me in the comments, call me sick but please just try and give me any advice. I feel so alone. I’m so ashamed by cheating that I can’t talk to anyone about it cause I feel so much guilt. I have nobody to talk to about it because people will always judge and talk and I can’t deal with that. I’ve put on a happy face but I’m not too sure how long I can do this for. Please help me.