I think I loosen up too much HU.

I understand that you've got to adapt to your opponent, but I find myself up against a lot of players who don't seem very willing to commit much more than a c-bet without at least top pair.

I try to crank up the aggression vs them, but I end up spewing, and they're often tight enough that they still won't pay me light when I actually get a decent hand. I get stacked going in with top pair / 2pr constantly. I've tried playing tighter, but then when I do get something, I never get action.

Is it typical for opponents to play just barely looser postflop than you'd expect in full ring? I expected looser postflop play to be typical, but that hasn't been my experience so far.

Does my table selection just suck?

Click sit out until nit leaves.

against this type of villian you need to up the aggression in a very specific way. cbet 90% of the time. Raise the button 95% of the time. Fold almost every big blinds, polarize your 3-bet range, and never ever spew turns and rivers.

Then these people just bleed money slowly but surely.

Are you usually joining a table with someone already sitting or do you start you own table? The people who are already sitting at tables are mostly regs, you need to start your own tables to let the fish come to you. Don't run the short stackers off either, they are there to give you there short stacks.