Cheating, Do I leave this 20 year marriage or do I stay? Help?

We have been together since 1993. since high school. since then it has always been rocky. we have to sons. He cheated than I cheated. He cheated on me when i was pregnant with our now 19 year old son, and I know the cheating has been going on for about 20 years and i just know about a few. 

The only one that has hurt me the most is him sleeping with my first cousin. I found out about nine months ago. 
I chose to forgive him, but I think about it when I'm driving, cleaning, watching tv, even when we are having relations I think of it, is this normal? I think if the how, when, how many times, where, etc. There has been many mean things he has done to me in the past 20 years that i also hate him for. When i found out about him and my cousin my life was turned upside down. I was sick to my stomach....I don't wish that feeling on no one. He then begin to tell me that he loved me and that i was the only one he wanted to be with and that i completed his cirle and he wanted to grow old with me, yeah all this becuase he got caught I know...

some how I still tried to work it out. Now I am so over this relationship. My older son is giving me a hard time becuase i want to leave and that hurts me so much, I dont' know what to do? help. do i stay for my kids until they have thier life or do I start a new one now? omg i want to cry just writing this.

believe it or not my kids no nothing about him cheating. at least i have not told them. I don't tell them nothing. we dont arue infront of them. we argue on the phone it crazy but we do...I think thats why my son is giving me a hard time, becuase in his eyes his dad does nothing wrong. I wish i can just tell him sometimes, but i don't want my kids to remember me talking bad about thier dad. He is a great dad to them they have everyting and some. And antoher thing I am very financially stable I don't need his money I have a great job, I own the house we live in. I guess i know the answer to my question I just want someone to tell me that its ok to feel bad for turning my kids lifes upside down and wanting to be happy. My house is not disfuncinal. My marraige is disfuncinal. I guess it is true when they say we don't know what goes on behind closed doors

Culled from: A yahoo answers question