Here's your answer: Exhibit one:

It's been a long time for me

It happens. No action for a while, you bust early, not a huge deal.

but it's no real excuse

Uhm, yeah it is. It's actually a very acceptable reason to come quickly.

I just feel like utter crap.

That's because "it's been a while for me". If you were getting laid left and right and just prematurely came with one girl who reacted like this, do you think you'd give 2 fucks? No, you'd laugh about it and think "That girl's a bitch, who cares". But since you're somewhat desperate to keep this arrangement going, you feel shitty because hey she jerked you off for a bit and you busted right away.

Your real downfall was the low amount of confidence in yourself. You said it yourself:

in hindseight, I probably made a bigger deal of it than I should have). She looked annoyed (obviously)....." I tried to keep going but she just sighed and said, "Don't worry...I'm not in the mood anymore." Although, that's probably because I was doing a bad job.

So you did well by trying to keep going after you came but you admit you made a big deal about it. That's a no-no. Even when you fuck up or do something embarassing or clumsy during sex, you never apologize for it. Have a laugh but always act like everything is okay, and that you aren't phased at all by whatever she might think. If she sees that you dont have confidence in yourself, her pussy dries up like the desert and it's all over. Hence why she stopped you from trying to keep going and said "I'm not in the mood".

So while everyone is saying she's a bitch and bla bla bla, which is a little true, really look at it from her point of view:

You had a talk about becoming FWB, so she's expecting that if you're sitting there arranging this kind of thing with her, that you're ready to fuck. She's obviously horny. So then she jumps on you and says lets fuck, and okay you're a bit disappointing in that you came really fast. Instead of just saying "My god you dirty little [whatever, slut, girl, whore], you made me cum like its prom night goddam.... come here now i wanna play with this beautiful little pussy" and then use your hands on her, give her oral, etc... eventually you'd probably get hard again and you'd fuck her afterwards. See the difference? You address your early climax but put a positive spin on it, and then you don't ask her to continue, you just do what you want to do to her. You dont assume youre doing a bad job, you dont show her that youre embarassed about whatever went down. You just continue to take her sexually with the next thing on the list if your dick needs a breather.

The reason you're taking all of this so hard is just because youre on a cold streak. If shes ignoring you now, you can try one last message to her, I'd say be a little assertive and tell her what you want to do to her and how you didnt get your chance last time. If she ignores you, forget about her forever and find a new FWB, and plow ahead. Shes just one horny chick in a sea of them, the real trick is to develop the right attitude and not to obsess over one missed opportunity.