1 month ago I cheated on the girl I had envisioned the rest of my life with. 

I was with her for 9 months, and for the first 6 months, all of our boxes were ticked when it came to one another. Sexual attraction, similar values and life goals. However, after the 6-month mark, I felt a palpable drop in sexual desire from myself. For no particular reason. This should have heralded the end of the relationship, but I continued to contain this deficiency within myself. Truthfully, this girl fit into my future vision seamlessly: tremendously kind, intelligent and highly-educated, wanted marriage and children, and got along famously with my friends. I did not want to lose all that for the lack of one thing. It's a crazy thought. My therapist (yes, I started seeing one after the betrayal) stated the obvious and told me that sexual desire is one of the most important things that keeps a relationship alive. I guess I just didn't want to admit that to myself or my now ex-girlfriend at the time from fear of sounding superficial. I know she felt my interest slipping away after the 6-month point, as I would have sex with her less, or even proactively show interest less.

Anyway, fast forward to the catastrophic end. I had a female co-worker for drinks in my flat (not a wise nor respectful decision, I know). But I did not hide this from my girlfriend. I told her who it was and that it was only us two. In my head, I felt I had the resolve to never cheat. As the night went on, and after a few drinks, this girl just kissed me. I gave in to temptation and we slept with each other shortly after. I kept thinking that if I just get this sexual fulfillment out of my system one time, then I could return and be a good boyfriend to my loving girlfriend from now on. Foolish. 

I drove to her house immediately the next day, and I told her I cheated on her, albeit it took me several hours to get it out. I remain haunted by her tears and heartbreak every waking moment. Her cries when she told me, "you were supposed to be the love of my life". What followed was a flurry of, "what we had was not special" and "I deserve the boyfriend that you said that you would be". I have no idea how long she was harboring these thoughts. 

In any case, I was a coward. I should have told her about my loss of feelings. And yes, I would have been the bad guy. But at least I would've been able to let her know that it wasn't anything she did wrong that made me lose attraction. I hate that I am the cause of someone so good thinking that there is a fault within them. 

I feel as if the only penance I can give is to ensure that my life never amounts to anything. That I do not deserve happiness after I did wrong to someone who made no errors. I have been in a state of self-sabotage since the day I told her, and can't see a way out. In addition, a mutual friend of ours, rightfully siding with her, openly expressed his loathing for me. We run in the same friendship circle, and I'm not sure my friendships with anyone will be the same anymore. I am the cheater.