I know there are TONS of posts about sleep on here and everyone is going to hate me for bringing this topic up yet again, but I need some advice b/c I'm at a loss... I also know that the first year is nothing but sleep changes, but I just need others thoughts on this and what I should try.
So LO hasn't always been the greatest sleeper in the world, but after we sleep trained, it got a lot better. (Yes, we did sleep training a little early and I know lots of people say it shouldn't be done until at least 6 months... not what I'm getting at here). Well, at about 4 months, she went through a small sleep regression. Lasted about a week. By the end of the week, we let her CIO for a night or two and she was good to go again. Now, when she turned 5 months (she'll be 6 months in 2 weeks), we hit another one and this one is hanging around. I started feeding her when she would wake up (though, not immediately, I'd give her a chance to go back to sleep). Then I started letting her CIO again and would end up having to go up and feed her anyway after about an hour or so of on and off crying. She'd look like she was going back to sleep then start crying again. So we thought maybe it was a regression at first... then we thought maybe she wasn't getting enough to eat in the evenings (my supply has been slowly dropping the last month or two and we've been supplementing with formula during the day, but I'd solely nurse in the evenings), so we started giving her some extra formula in the evenings. Seemed to work for a night, but now we're back to lots of crying.
Anyway, to the point... maybe it's a sleep regression, maybe it's teeth, maybe it's a growth spurt (though, I thought those only last a couple of days)... but I'm just not sure how to handle her nighttime wakings. Should I go in and nurse her anyway? I'm just afraid that's going to start a bad habit (which I think has kind of happened already), but we try to let her CIO and we all end up awake for 1-2 hours in the middle of the night and she ends up getting nursed anyway. Hubby tries to go in and soothe her, but it seldom works. She knows how to put herself to sleep, so I know it's not that she needs help with that. Though, maybe she does if she's going through a sleep regression. I am just one tired mama and am barely functioning! I don't know what else to try!!

Re: the sleep topic again... need advice
Since he is nearing 6 months old I was thinking about doing CIO if he woke up before the 6 hour mark because I know he can go that long without eating. CIO isn't meant to eliminate night feedings - just to help learn to fall asleep on his own. But it's hard. I have tremendous guilt hearing him cry and getting himself really awake when I know I can nurse him back to sleep in a few minutes. So I don't know what to do. I just don't think there is anything I *can* do, I've done it all. My baby knows how to fall asleep on his own. He just doesn't *stay* asleep. I know we're going through a Wonder Week right now and I'd like to blame our lag in the progress we were making on that but I have zero hope that things will improve. I'm just trying to accept it. It sucks feeling so tired everyday, I feel like I'm hungover! It's awful.
Other things to consider:
Is the room hot?
Teething?
Dirty diaper?
Does she need white noise?
Formula intolerance?
Rash?
Hair wrapped on a body part?
Since birth, I've just immediately nursed him when he starts to cry in the night. He's still in our room so I'm only awake for 10 mins tops.
Thanks for the advice!