As a (very occasional) writer, I do try to thank people for feedback, but I always feel so intimidated by my own feedback, and never know what to say.

Why do you think this is? I have seen a couple of people say that and it just baffles me. Most people would agree that the polite/civil/nice/whatever response to feedback (whatever analogy you might want to use for it) is to say thank you. I have never found that to be an intimidating thing to say. I might not ever know what to add after that, but acknowledgement of the feedback you've recieved is all that's really necessary in most cases.