Professor to whom I broke my promise is now a collaborator. What to say?

I understand that this situation is somewhat awkward, but I think it is important not to overthink this. It was, after all, a fairly long time okay, and at the time the professor said he understands, so you may be taking this incident harder than he is.

But what should I say to X to break the ice?

I probably wouldn't mention anything, at least not at the beginning and definitely not over e-mail. Bring it up at an opportune time, or not at all.

Should I start by apologizing again?

No. See above. Even if he remembers you (which is not at all guaranteed), starting a collaboration with "remember how I reneged from our agreement last time?" seems supremely awkward. Should the collaboration take off and you meet more often and personally, it might be nice to apologize again, optimally in a social setting. Alternatively, if he brings the topic up, you can and should apologize. However, don't make a big deal out of it if he doesn't.

Should I just pretend that I don't remember about him? This is good if he already forget about me. But if he still remembers about me because I caused him so much trouble, then I will appear as a horrible person to him.

You don't need to pretend you don't remember him. Just interact with him normally - even if you don't start a conversation about your last interaction immediately (and he remembers you), he will probably not assume that you forgot about him, but that you are trying to not make the situation awkward. He won't consider you a horrible person for this.

How to break the ice? You don't need to mention the issue, much less apologize when you first meet him. Just act like with any other professional: "Hi, how are you? It looks like we have some exciting research ahead".

You are talking about 7 years ago and the professor might not even give it any importance, otherwise he'd not be willing to collaborate with you. So I'd suggest you stop punishing yourself, get over it, and just focus on acting as a professional.

If the issue arises, make some funny comment and move on. If he insists on making it personal and taking you to a guilt-trip or you feel uncomfortable working with him, just stop the collaboration and find another colleague to collaborate with.

You've already apologized, the apology was accepted, and you've both moved on.

There is no reason to bring it back up, and the professional thing to do is to let the past stay in the past. Chances are good he will as well.

Unless he brings it up, don't bring it up yourself. Conduct your research as well as you can, and show him how you've grown over the years.

If it still really bothers you, then well after you've established your research relationship, at a time outside of work (social), you might simply say, "I still feel bad about..." and express your feelings again, adding, "But I'm so glad we have this chance to work together."