Not ready for anything sexual?

I've been dating my boyfriend just about a year and he likes to touch my breasts, underneath the shirt. He's only done it twice because the first time I freaked out on him and the second time I just kind of let him because I felt bad about the first time. But now I've realized I'm really uncomfortable with it and I feel really embarrassed because my boobs are really small.
The problem is is that I don't know how I would tell him nicely I don't want him to do that. I feel bad telling him to stop because he always seems so much happier when he does it, like right after he turns into a happier person in the relationship, telling me how much he loves me and he just seems happier overall.
What should I do? I don't know what I should do, but I'm not ready and I'm scared it will turn into more and I don't want to lose him because I truly love him.
Not ready for anything sexual?
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