What to do when your boyfriend is rethinking the relationship?

My boyfriend has pulled away from me although he acts casual when I contact him, I can tell he's not the same. I am pretty sure he is rethinking the relationship and might break up with me soon. I did the wrong thing and have been texting him trying to find out what's wrong and trying to get him to call me. He's become avoidant. What can I do to prevent a break up? Sadly I'm afraid it's going to happen :[
What to do when your boyfriend is rethinking the relationship?
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