All the nits? Aren't you the one complaining about all the fish doing crazy stuff all the time?
But in all seriousness, I have a few suggestions:
1) Postflop, bet fairly small all three streets (but bigger on the turn and river with the nuts to get stacks all-in). Fold to any and all raises without the absolute nuts. You have 2nd nut flush? Fold to the raise. Have top set on a board where a straight is possible? Fold to the raise.
I haven't actually tried this myself, but it's something I was contemplating doing, as I also haven't had much luck with nl10 (and even nl5) lately. Not necessarily as a permanent solution, but to see if the winrate increases when we stop calling so much. Because if they (in general) are nitty, we probably shouldn't be too interested in stacking off without the nuts. Even if they have semi-bluffs, their range as a whole is really strong.
If you find yourself calling off a lot with great (but not nut) hands and finding they often have better, maybe try this out. Sure, it's a very nitty idea and will require discipline on your part. But, if you're out of ideas, you have nothing to lose by trying this out for awhile and seeing how it goes.
EDIT: I should mention this is for regs/nits/tighter players. Against the bigger fish, not so much. Just depends how passive they are I suppose lol.
2) Tighten your ranges preflop. Go through your database and search for when you RFI (for example -- you can do the same for other things like 3betting, calling RFI, etc.). Then look at how individual hands (or subgroups of hands) do. If you're opening UTG with mid suited aces, for example, and you find that you're losing money with them, cut them out. Just open with your profitable hands for awhile and see how it goes.
Again, might not be a long-term solution, but might help you get back on track to winning more. Maybe if people see you start playing nittier, they might adjust. But maybe not. And if they do, they might not adjust properly. Just another thing to try out.
3) Switch to zoom 6-max full-time for now. Ok, so I don't know if this will actually help. But I do know it's possible to at least beat nl2z 6max for a nice winrate. I can't really speak for nl5z/10z, except to say I've at least been beating nl10z over a close to 100k hand sample. So if I can do it... hahaha. NL5z on the other hand, it seems I've been struggling a lot more with. Maybe 5z is a lot nittier.
4) Check your database again for how often you're defending against each position's RFI. And I mean how frequently you're defending from all positions combined.
So for example, versus a 3bb UTG open-raise, maybe you defend 10% from each position, from MP to BB (just an example to make it simple). That means you're folding from all positions 59% (90% x 90% x 90% x 90% x 90%). So you're defending 41%. MDF against a 3bb size would be about 33%. So we are defending enough.
If you work this out, and you find you're not defending enough, then maybe to look to call/3bet more from some/all positions.
5) 3bet more, with a more merged range. 3bet with high/mid pocket pairs, higher suited/unsuited aces, higher suited broadways, etc. 3bet less with low/mid suited connectors and small suited aces. Something I also started experimenting with was 3betting 4x with premium pairs and AKs/AKo/AQs (exact hands vary according to positions), and then 3x with the lower end of the spectrum.
Those are just some ideas you can try, or not try. Just thought I'd throw them out there; I don't know if you'll find any of that helpful. I hope you don't give up though, and keep going at it. Just look at how long I've been at the micros lol.