Thank you for the responses. To clarify, I am sad to admit this is something that is very much in the past.

My girlfriend is not the type to flirt with other guys and she has made me multiple paintings without me even asking. As far as him being in her life, when I mentioned that him being facebook friends and being able to spy on her bothered me, she removed him immediately, no questions asked. Also, she said that even though she initially agreed to make him a painting, she later realized that she would hate if I did that to her so that's why she ended up not doing it.
Also, my girlfriend is a VERY giving person and struggles to say no. I simply have an inability to relate. I'm a giving person, but I would never do an entire project for someone I used to have sex with while falling in love with someone else. Alas, it is in the past and this is where I struggle.

This is what makes me feel the most guilty. It's clearly a matter of forgiveness for something that happened so long ago. I just struggle with it a lot and it makes me feel horrible about myself.

Part of my forgiveness problems stem from the fact that I have obsessive compulsive disorder (genetic trait, everyone in my family has it). Naturally, specific thoughts (from the past) stick in my brain very easily. Thus, what is in the past feels to me like it's in the present. So even though my GF treats me like complete gold in the present, my brain has the tendency to obsess on particular thoughts from the past. Again, I am not proud of this and I want more than anything to not be this way.

If anyone has any additional thoughts, I would appreciate it. Thanks again!