Need some advise as to how to best end a relationship. Here is some back ground:

about 18 months ago I split up from my wife of 18 month's who I had been with for around 8 years, this was not a good time for me as I didn't really understand why we had split up but I got over it and started living my life again, after six month's I met my current girlfriend, I moved in with her after about 6 month's everything was going really well and felt really lucky to have had a great relationship with my wife and it un-fortunatley ending to then find another wonderful girl, life was good.
Can you guess what happens next? That's right my wife decides that she has made a mistake and wants to make another go of things, which casts doubt over things with me and my girlfriend and I end up moving out for a couple of weeks, eventually I decide that I should stand by my promises to my girlfriend (being that I said I would not go back to my wife), but ever since I can't help but doubt my decision, I have come to a point where I don't really want to be with my girlfriend anymore and not because of my wife, but because I don't want to be with anyone at the moment I am really confused and just want to enjoy my life with my friends and with out the pressures of a girlfriend, but I feel so bad about hurting someone who has stood by me through all this, I know I should end the relationship but don't know how? I want to make things as painless for my girlfriend as I can.