Ugh, that feeling really sucks... We're often our own harshest critics but I know remembering that doesn't make this headspace any less shitty to sit in. I don't know if this helps, but something I've recently learned (after years of feeling similarly) is that there's a difference between being too hard on yourself and fussing too much VERSUS genuinely not being given enough time to work on something.

Your experience sounds like a great opportunity, but if you're constantly hauling ass and this is a recurring issue that weighs on you, the schedules themselves might be worth revisiting. Consider asking for more time if you can. It doesn't hurt to ask. If you can buy yourself a week or two to invest in strengthening your script, it's going to do wonders for your mental health not only through the making-of but, y'know, after it's released and there's nothing else you can do about it. If you can't get a nudge on the deadline... Should you keep taking projects that you now know don't allow you enough time?

That's the other thing! There's nothing else you can do about it. Learn from your experience and work smarter next time. As much as you dislike this project, it did something valuable because you know better now. That's what I tell myself so I can stop dwelling, anyway.