Why is it so hard to accept the fact I'm a lesbian?

It can be hard for different reasons. Sometimes because you grew up in a conservative, strict household. Sometimes because it is still foreign and you have to make some experiences. The first step is educate yourself, be self-aware and embrace your sexuality. It might help talking with people/friends about it to get used to it and learn how to cope with the new situation. Btw sometimes people refuse to embrace their sexuality because they are not in a safe environment where coming out of the closet might have severe consequences. So try to figure out why and then work on the how. Celebrating pride month is a good way to embrace your sexuality and yourself as a person, so maybe you wanna give it a try.