I feel some shame too: that I was so stupid with the contraception (it was "just one time" but of course I know thats all it takes.
Sorry but this makes me think the contraception is condoms. If so, you didn’t screw that up alone; you both did. He is equally responsible and I don’t think it’s right that you are suffering through this alone while he is in blissful ignorance of the consequences of his actions.
If you missed a pill, I’d be concerned that you couldn’t say to him ‘Oh I had a stomach bug so we’ll have to go careful this week and use additional / do other stuff’. Is he generally judgemental, you mention this fear a couple of times?
Regardless of the method…
Personally in an established relationship like this, about to move in together, I would tell him. I’d want the support. I would personally have the abortion, FWIW.
I wonder if your worried he might try and persuade you to keep the baby. You seem to have learned you don’t want children after all. This could be the catalyst that ends the relationship if it shows you actually want different things. Is that really why you don’t want to tell him?
Wishing you strength either way.