I'm dating this certain girl now for a few weeks already, and really took much time to build a huge amount of comfort with her. Unfortunately she's one of those insecure girls that would need a TREMENDOUS amount of comfort to feel good about having sex.
Actually she told me that in the past she never felt good, comfortable, relaxed with a new partner, she only could relax enough to have sex when she's drunk.

Soooo, I really spent a lot of time with her, did everything to build comfort, but last night at some point I thought that "2 Options, either I'm gonna fuck her NOW, or I'm gonna dump her because there's too much work involved". So I f-closed her, and surprisingly once I broke down her barriers like a bulldozer she was really into it and enjoyed it.

BUT this morning she was a typical case of buyers remorse, I could clearly tell that she's feeling bad about it. Luckily this girl is totally into me, mainly because I really took my time to build enough attraction, so she's not flaking, but she's feeling bad about it, and I'm looking for a way to ease her buyers remorse.

Any ideas?

What you did was basically convince her to have sex without consequences. She's not getting pregnant and not being called a slut, there's nothing she can be worried about now. However, she somewhat feels the effects of what it would be like with a consequence and she's making a reality out of it when in fact, it isn't there. It's just a fantasy all up in her head. What YOU have to do is not react over it. Just act normal and go on with your life. If you try to comfort her for something that isn't there (not being pregnant/getting called a slut) then her fantasy will become reality. She will sense that there IS something wrong. So don't go about it as there IS something wrong as you are now. Stay chill if you want to get sexual with her again. If you keep this up, she will eventually not be insecure anymore.

Sounds like she hasn't had that much sex in her life yet, her insecurity is due to inexperience.

I'm actually following the "don't react" path, but somehow I'm afraid if I'm not able to speed things up with her then I'd have to break up with her. As things are going right now it could take months till a reasonable sex-relationship would be established, I'm not really in the mood to wait that long

Meanwhile (2 days later) she's messaging me all the time that she wants to see me again, so I'm actually thinking about a way to "implant" a thought in her that says something like "if I'm having more and better sex with him we'll be seeing each other more often".
Obviously that has to be done in a way so it doesn't look like I'm extorting her ala "gimme sex or we don't see each other anymore".

BTW, I know this sounds evil, but I give A LOT back to her, and I'm honest with her