Hi there anonymous13234, I hope it's okay to jump in and share some thoughts here.
I don't think that these moments where you're feeling an urge to make yourself attractive to men mean you aren't a lesbian. Regardless of sexual orientation, most women get a lot of messaging about the importance of being attractive to men, and it can be hard to drop those impulses and behaviors even if you know that you don't actually want to date any of those men! You say that you'd rather be straight sometimes, and I wonder if some of these behaviors aren't an attempt to kind of push or force yourself into what you feel you "should" do or would be doing if you were straight. Whatever the reason, though, you sound pretty clear that you aren't actually interested in these men.
In terms of your fantasies, I agree with Sam that having sexual fantasies that focus on men doesn't exclude you from being a lesbian! It even sounds like some of these fantasies are pretty disconnected from the gender of the person involved at all, and more focused on the shape and size of their body. It's not always possible to find fanfiction or other content that's exactly
tailored to every preference, so it doesn't seem odd to me that you'd find yourself reading content focused on men even though you don't find yourself attracted to men in general. And just to emphasize something else Sam said: it's pretty common for lesbians to read/write/otherwise focus on gay male content in fandom spaces! I have several friends who fall into that category.
You mention having some guilt about being gay, and wishing you were straight instead; are these feelings something you've ever talked with your friends about? You mention not wanting to get into the details of your fantasies with them, but do you think they'd be able to give you some affirmation around that aspect of things?