well my DS1 was a terrible sleeper, so I think I've had various bouts of good sleep but for the most part have not enjoyed reliably STTN for nearly four years.
I have done Ferber. Worked great for both kids. Until something happened to disrupt it. There is definitely a difference between what I think of as "complaining crying" and "real crying" and it's something I learned through doing CIO. So if DS2 is just complaining, I know he'll fall asleep. If he's really crying, I go to him.
For a while after sleep training he'd go down at 7:30 and sleep till 4:30 or 5 every night. it was awesome. I'm totally fine with nursing him at 4:30 or 5.
Lately (for the last 6 weeks maybe?) he's been waking erratically and sometimes complaining and sometimes freaking out. for the last month or so I've taken to nursing him again in the middle of the night, both for my supply and his comfort. But sometimes he wakes up at 9:30, or 11:30 and it's like having a newborn again.
Between the teething, eczema, milestones and potential for illnesses I'm unwilling to really let him CIO right now because I'm always worried something is really going on. But I'm at the end of my rope and the lack of sleep is starting to affect me negatively in my health, in my work, and in my emotional state.
I know these days are short (believe me I know, I look at my 3yo and I can't BELIEVE he's so big already!) but sometimes I feel like I'm in too much of a fog to even enjoy them properly. I will not miss this lack of sleep part.
ETA: sorry this was less advice and more commiseration. my only advice is that all things pass - the good and the bad. if you don't feel right doing any CIO approach, know that he will eventually sleep. my DS1 was horrible but he's ok now. we lie down with him when he needs us but it's not like it was. now he's the good sleeper! haha. GL!