“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. This is how you explain your lack of will power.
I can't control myself when it comes to chocolate.
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When you "can't control yourself", it means that you act based on your feelings instead of your thoughts. You can use this phrase to talk about doing something that you shouldn't do:
When he said that, I couldn't control myself. I jumped forward and punched him right in his face.
But you can also talk about something positive:
When I hear that kind of music, I can't control myself. I have to dance!
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.
"When it comes to ___" can go at the beginning of a sentence or at the end:
When it comes to eating, nobody beats Mako!
I'm pretty much worthless when it comes to computers.