I'm going through a divorce and I met this guy who is married and I like him a lot. I want to stop this because the wife is on to us but I can't stop thinking about him. I want to end this now but don't know how.

Think of the wife's pain.

Think back to a time in your life that hurt you so much. Times that by 50 and should give you some indication of what the wife would feel.

What would you feel inside, if because the wife couldn't take the pain in her heart any longer because of the betrayal of her husband, she tried to harm herself or worse.

You could never live with that. Honey it's not worth it.

You may still not be healed from your divorce and things of the past. Take a break and enjoy your freedom and I bet it will be then that you will meet a nice guy, a good guy, who is available and would never cheat on you.

The pain it causes is just not worth it. End this chapter and move on.

Okay?

How do I make myself stop thinking of him?

Distract yourself as much as possible. I understand sweetheart I do and I am not judging you. You are worth more then playing 2nd fiddle.

There is a thread here entitled "Realtionships" at the top of it are "Stickees" that
Help getting over someone you care about. They are very helpful.

If you can't find it, let me know, I will cut and paste them for you.

Here's a hug... and your are worth more then 2nd prize, but I do understand a sad heart.

Go find someone you can have all the time, this is leading to something bad, you have to put yourself in her shoes as well and think how you would feel...

Stop emailing him, don't see him, don't call, don't answer his calls and so on

If he has stopped communication with you then he may get irritated if you continue to try and contact him. He could even try and press harassment charges if he feels that you are messing things up with trying to work things out with his wife.
Get out and socialize meet guys, join clubs, organizations, health spas, take a class of some kind, do whatever to get your thoughts on you and your future. Put him in the past.

Make a list off all the bad things about him and about you seeing him and read it to yourself every day. When you get up, tell yourself "I deserve a relationship where I can be #1". Like NOhelp4u said, get involved in activities. The busier you are, the less you will have time to think of him.

Bottom line is he is not available. You have no right positioning yourself in his marriage. Find someone that can focus on you and only you.
You can't stop thinking about him because he is a "forbidden fruit"