Why can't I talk dirty to my boyfriend?

I'm in a long distance relationship so my boyfriend and I don't get to see each other very often. We keep it interesting by sending nudes/videos but for some reason, I just can't talk dirty to him on the phone. It makes me so uncomfortable. I just can't help but feel like I sound stupid. I barely know what to say too. And my boyfriend is so sweet and tells me not to worry about it but it makes me feel bad. So I just need advice on how I can get over my fear because I really want to do this for him.
Why can't I talk dirty to my boyfriend?
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