Hey. I've had a few serious relationships.
The last one was the most serious (but not the longest), it lasted 5 years.
It was a co-dependent toxic as fuck relationship between a girl with BLPD and a guy (me) with a whole plethora of issues.
You wouldn't know this to look at us. Everyone assumed we were super happy in our relationship. I was a semi pro athlete, she took interest in my sports and training and took to it too, we both had masters degrees and we both had very good jobs, etc.
When we were / are single. We excel, but in relationships we're pretty bad.
In the 5 years I probably cheated on her 50 times. All 1 night stands or internet hook ups. All worthless, all worse than her, all less hot than her (to me), all forgetable. In fact, I either can't remember, or never knew the names of most of them.
In any event, I cheated because it temproarily refueled me for the very exhausting relationship. Now without getting all bawwww bout it. When I was young I was abused, I went off on a tragectory that wasn't really mine. To everyone else I had a solid identity, to me, I didn't but didn't even know it. I've been in counselling / therapy for 3 years now. Essential stuff. Anyone who doesn't do it is the emotional / intellectual equivelent of a fat fuck who never went into the gym, or worse, a skinny person who never needs to go to the gym... pure self delusion.
Now, regards OPs perdicament. Here's what I know. I wish I had abandoned the relationship at many stages - before it even began. But at least I should have abandoned it when I felt like cheating.
Now, maybe she's perfect, and maybe the relationship is perfect... well if that's the case, you're the problem, get out, get into therapy, become a man.
If she's the problem, get out, get into a new, better life, then a new better relationship.
But if you're cheating it's because you're fucked. Sort yourself out.
But the first thing you need to do, is read the book The Little Prince.