Hi! I'm a month and a half ahead of you, and while I can only talk from my own experience of co-sleeping, at 4.5 months my little one went through a growth spurt and sleep regression.
The problem at this age is that they can become highly distractable during the day and consequently feed less, so they end up reverse cycling (feeding more through the night than the day). The best way to try and remedy that is to remove all distractions during daytime feeds - move to a quiet, dimly-lit room and see if that makes any difference. The other thing to bear in mind with respect to feeding and growth spurts is that your prolactin levels peak during the night, so often it's an attempt to boost your milk supply. I wouldn't class it as bad habits, per se, it's more down to developmental and growth phases. We also had issues because my son started rolling at 4 months and used his night waking as an opportunity to practice! It did eventually cease to be a problem, but its obviously a very exciting time for them when they learn a new skill and they have no concept that night time is not the time to be doing these things!
The not so great news is that because of the sleep regression, he is still unable to link sleep cycles and pretty much wakes constantly for feeding to help him get back to sleep. Doesn't necessarily mean your LO will be the same, just worth bearing in mind.
Like you, I got worried about the self-soothing issue, but I have recently come around to thinking that it's a heap of nonsense. Regardless of how long it takes them to get there, they WILL eventually learn to link sleep cycles and go to sleep without needing us. I am 100% with you on the exhaustion and it being tough, but having sat down and thought long and hard about it, any kind of "training" them out of it is effectively denying them what their bodies are biologically programmed to need from us. If they didn't need it, they wouldn't ask it of us. With that in mind I thought about how I could find a way of gently trying to adjust things (my LO refuses to sleep without me or the boob, meaning he often wouldn't sleep until 10-11), so the other night I decided, since I was exhausted, I would lie there and feed him for 3 hours or so from about 6:30. He did still wake through the night, yes, but he was far less wriggly and by my own usual bedtime I was able to move away from him and sleep comfortably until he did wake for another feeding session. Obviously one night doesn't mean I've found a solution, but my point being that I think we just have to try many, many different things until we find something that helps.
As for the sore nipples, make sure you are piling on the Lansinoh after every feed.
Appreciate I haven't given you all the answers you might feel you needed but I at least hope it provides you with the reassurance that this is all very normal and that there's nothing you are doing wrong. Keep up the good work, mama! X