Why do I keep doing this

I did it to myself again….how many times is this? 20 times in 6 months? I have gone back and forth for weeks now with my current N. He has done some major damage. I didn’t realize that I was being triangulated. That he was seeing his ex and me at the same time. He was so good and convincing me nothing was going on with them, and that they only communicate about her 2 year old daughter (that calls him daddy since he raised her from 6 months). He has convinced me, or should I say brainwashed me that there is nothing going on. So last Thursday I went over to his house, he said the keys were in his car and to let myself in….I get there, doors to car are locked and in the dashboard are 4 pictures of him and his ex, at a wedding, playful and kissing in one of the pics. I saw the proof with my own 2 eyes, and guess what….he tried to convince me that I didn’t see what I saw. Holy Crap. That we weren’t talking at the time of that wedding, so it doesn’t count…but yet it’s on his dashboard currently and we are supposed to be back on track.
I need help to get away from him. I am back on day 1 of NC. Everytime I try to get away, he pulls me back. I have to stay strong, I have a 4 year old son who needs a healthy mother. This man is slowing killing my soul, but the sad truth, I am letting him. What makes him so wonderful? Nothing…..He has no job, no car, sleeps on a friend’s couch, no home, doesn’t see his real child (6 yr old son) and even admitted he treats the 2 year old better than his own flesh and blood. He lies about EVERYTHING!!! What keeps me going back and why can’t I just stay strong?!?!? God I just want this pain to go away already, I cry all the time, and can’t focus on anything at all. Work, home, my son, family, friends, I cannot get him out of my head, and he is living there for free.