How can I overcome my jealousy issue?

You have to come to terms with one path or the other. You seem to know he's a great friend, not boyfriend material, right? Because multiple times you've realized you don't really like him that way.

So embrace being his true friend. Which means, you WANT the best possible girlfriend for him. You want him to be happy, and to find the right partner. You don't want to control him do you? Or for him to be lonely? Good friends feel happiness WHEN their friends get happy, and support their true friends WHEN their friends need support. Think like that, and you'll LIKE hearing about his next girlfriend, because he'll be all into her and you'll see him get happy and all that, and that will make you happy. Then they'll break up (NOT because of you because remember you treated him like a true friend the whole time) and you can be there to help him pick up the pieces as a true friend should.

OR, admit to yourself that you're actually "into" this guy and quit screwing with him and dumping a few weeks after going out. Instead go out, and really connect with him. Instead of bolting when you have a doubt, TALK TO HIM about the doubt. NOT "I don't like you anymore" but "Back when we were just friends, we used to have fun doing *insert the activity you miss here. * But we don't do that now. Let's try doing that again. But as a couple this time. " See? Turn TO him and build a stronger and stronger relationship, rather than turn away from him and break this poor guy's heart yet again.