The first thing to do is to not feel bad about it!
The second is to just do some for yourself. Take posting it completely off the table. Use a prompt list (like THIS one or THIS one) go to random.org and have it pick for you, then give yourself permission to just write. For a hundred words, or two hundred, or a set amount of time, though I wouldn’t go any longer than 20 minutes. Write whatever. Don’t edit. It doesn’t even have to make sense or be an attempt at a story. Get the words out. I still sometimes start days like this when nothing wants to flow. And shove these bits into a corner of your hard drive. They’ve done what they’re supposed to, and don’t need to be anything more.
From there, when you feel ready, start writing one-shots, of whatever length you like, but focus them around some idea that you have. It doesn’t need to be a grand one. (You OTP goes to dinner and a movie!) These also don’t have to ever see the light of day if you don’t want them too.
And go from there. Basically, give yourself permission to write, to not finish, to do whatever you want to with the words. The more you do it, the easier it gets, but I often think that imagining an audience really get in the way of writing. So take them out of the equation.
Also, work short for now. Writing a long fic with multi-chapters can be exhausting…and that’s when you’re totally excited about a concept and confident you can write it how you want to. If you’re feeling unsure, don’t set yourself up for failure. Write something 300, 500, 1K, that you can finish in a sitting. Post it or not depending on how you feel about it, but maybe save that decision for later.
Most importantly, enjoy the process. Write angst, smut, fluff, crack, whatever makes you happiest. (Again, toss out your imaginary audience, they don’t matter) Have FUN.
If you’re having a good time with it, the rest will flow!