Advice! What to do in this situation.?

Alright, to make a long story short I was in a lesbian relationship with a woman, we will name her Jen, for two years. It moved pretty fast, two months after we hooked up she was basically living with me. A year into the relationship we broke up for roughly four days. Before we got back together she told me she had had sexual relations with someone, of course I freaked out but she promised that she had not cheated on me. So another year went by and I started noticing all the signs to point to the fact that she was cheating on me and one night we had another blow up and she left me. A week later I discovered the truth about the three days, how she had an ongoing relationship that went on for months. And then I discovered that she had been cheating on me AGAIN, which was the real reason why she left me, to be with this other girl, we will call her Lisa. I actually introduced Lisa and Jen, because I worked with Lisa. We all actually hung out a couple of times. Anyway, Jen told me she was in love with Lisa and etc.A couple of weeks later, Jen asked me to go to the bar with her, so I did. We had a great time, felt like we were reconnecting. We kissed and she freaked, went and stayed with Lisa for a couple of nights and then moved to Pennsylvania with her family. This was in August of 2008. A month later she decided she wanted me back and promised she would change.Now here comes the difficult part. After she left, I hired a few new employees, one of them is a stud lesbian, we can call her Sam, and I totally wasn't looking for anything at that time since things were so hard with my love life already. Sam seemed to be in a healthy relationship as it was, but months passed and I really have connected with Sam and it turns out that she is miserable in her relationship and she confessed feelings for me. I would NEVER cheat, and Sam had said she would leave her relationship for me so that we can take it to the next level, but this involves me telling Jen that things will not work out, and I don't want to hurt her, I do love her very much. And I also can't help but feel extremely guilty for Sam's girlfriend, but I would LOVE to give Sam a chance because I feel so drawn to her. ADVICE? I don't have too many people I can talk to this about and I am lost in this situation. Advice would be awesome.

Sum up the entire thing in one sentence or pray to god because there truly isn't anyone that is going to sincerely read through that whole thing and analyze and form a conclusion and a reponse to your exact situation.