“I can't control myself when it comes to chocolate.”

You're supposed to be on a diet. Someone offers you a chocolate brownie, and you take it. While you're taking it, you explain why:

I can't control myself when it comes to chocolate.

(someone) can't control (themselves)

When you "can't control yourself", you act on your urges rather than your long-term plans. You do things that you know you shouldn't do.

when it comes to (something)

Saying "when it comes to ___" is a way to limit the topic of your sentence. For example:

James is a genius when it comes to computers.

This means that James is very smart, but specifically on the topic of computers. On other topics, James might be smart or maybe not.