You're getting a lot of "don't feel bad about it! it's totally ok!" replies, and they are right! It is totally ok, but the fact is you do feel bad and trying to just... not isn't easy.

Here's the best advice I can give you, as a woman who has had a few regrets of my own in my past:

First of all, you need to accept that you did this and there's no changing that. There is no sense in beating yourself up because there is literally nothing you can do to change the past.

Second, try to figure out what exactly is making you feel so guilty. For me personally it is just latent and deep rooted feelings that women are meant to be the "gatekeepers" of sex and by having sex I was failing that requirement, and therefore less worthy as a woman (note, that is a load of poppycock but it is how I felt at the time). Maybe it's fear of social stigmatization, or judgement or whatever but just identify what it is that's making you feel guilty about having sex. There is a lot of pressure and stigma around women and sex and that can be really hard to cope with.

Work through that. This is where you will get to the part of growing, learning, and changing. And finally you will be able to look back and say, that is a thing that happened and I learned and grew from it and here I am today and that is awesome. And you too can be one of the people saying "it's totally fine! it's not a big deal!"

Ultimately, sex is sex. That's it. We as a society have this whole bizarre relationship with sex wherein it is simultaneously of paramount importance and wound up with intense social implications. But at the end of the day it's just two people enjoying an intimate moment together. Sometimes it has more meaning to it than that, but it doesn't have to. You are fine. You are not damaged, you are not less of a woman, and you will have a complete life if that is what you want. Once you are able to accept it as not the end of the world you will find so many other people who are of the same mindset.