Why?

If you are happy in your marriage, why would you do things to hurt your partner? I don't get it! How in one breath can you say that you want to be with a person and then turn around and take flirting with some one else to a whole new level?
Recently my spouse has taken lying and flirting to a place where it makes me physically ill! I have seen that he has conversations with other women from his past on facebook. I told him how much this bothers me, and even went so far as to not even go on facebook myself. Any enjoyment I took from going online and playing the apps has been destroyed. Every time I went on, all these girls stalking his page was thrown in my face. He accused me of being paranoid and crazy. Now there is a history of him making it very clear to me that he cannot be just friends with a girl. I am by no means perfect, and have made mistakes myself. The last big mistake I made being two years ago. Yesterday he had wrote something sweet about me for the whole world to see, yet he made an error and asked me to go look at it. When I went online his facebook page was up along with a conversation he was having with some bitch from NY. The parts I got to see before he ripped the mouse out of my hand to delete it painted quite the picture!

Her: get your ass to work
Him: I'm home today
Her: I'll be right there

another part

Her: miss you
Him: miss you too
Her: love ya xoxo

This is only two little parts that I got to see. I can only imagine the rest of it! He tried to tell me that it was nothing. If that was the case wouldn't I have been able to read it? I know better! I'm not stupid!
After about an hour of fighting, and him lying, he finally admitted that "maybe I flirted too much, but I didn't think it was a big deal"... YOU FUCKING THINK SO?
Now I am stuck with what to do. It's one lie after another, and one heartbreak after another. I know the obvious is to kick his ass to the curb. What a fucking Jerry Springer wanna be am I!!

I got three kids to think about and 18 years of my life involved. What the hell do I do? When is I'm sorry said one too many times? When is it will never happen again supposed to be believed?

Not only is the lying and being verbally romantic with other women weighing heavy on my soul, but the whole issue of why did he do it. Was I not stimulating enough for him that I couldn't keep his interest? Or is he just an insecure sick fuck who is just never satisfied with what he has.

This whole situation is just so fucking horrid that I needed to vent some where. Any advice is appreciated. There is 18 years of back and forth fuck ups to be taken into consideration, but really for the nonsense to be still going on now is inane.