Am not going to belittle what you feel for him, but its probably one of the most unhealthy kind of loves there. I think you're addicted to him and to the situation. As bizarre as it sounds, this kind of relationship, for all its up and downs, is exciting. Problem is, its all the wrong kind of excitement.

You deserve to be with someone who is free to love you properly, not in moments that are snatched. You deserve to find someone who is with you 100%. And most certainly his girlfriend does not deserve to be cheated on, lied to and hurt. In my honest opinion, this man deserves neither of you ladies.

You ask for strength to walk away? Grow some and walk away. Treat yourself and the other lady better. Keep away from the man, ignore any contact. As time goes by, it will get easier, and the fog of addiction to him will lift, and you will probably see him for the greedy, selfish man he really is.