^ Don't kid yourself mate, if you are a heavy user, as you describe, and your bladder capacity is decreasing, then chances are it is the Ketamine. 28 is not old enough for this to be age related.
I would still love to hear your thoughts on my situation if you have any!
I wish that I had some useful advice to give you, unfortunately I probably don't... but since you ask, I have actually had a few thoughts, I have found myself thinking about this occasionally... I guess because I have also been kind of detoxing recently from a K binge at the beginning of the year and occasionally I do still worry maybe I went a bit far that time and did something permanent that is going to come back to bite me.
My first thought, or rather, question, is have you considered the possibility that your symptoms, if not entirely psychological, may have a psychological component that is prolonging your recovery?
I ask because I'm assuming you knew about the bladder dangers of Ketamine before you developed this problem, and it seems that it does not have much functional impact on your life. I don't mean to diminish what you are experiencing here, I should say neurological rather than psychological maybe (arguably, at some level, they are essentially the same thing) because obviously regardless of the source it is still a real issue that you would like to deal with. But, do you ever find that the pain or discomfort is sufficient that it actually distracts you from something you are doing? Or is it moreso something that comes on when you have few distractions, and are alone with your thoughts, so to speak?
Again, I'm not saying that I think this is the case, of course I really don't know, it just occurred to me that maybe this is a possibility worth considering because at the end of my own aforementioned binge, I had my first (mercifully quite brief) twinge of bladder symptoms myself, and even now I am still not totally decided if I was feeling something real and physical, or if I just created this weird feeling in my head and started to associate the feeling of needing to urinate with something more sinister.
During this period I would only ever feel any increased "bladder awareness" (for lack of a better word) when I was alone, doing something mindless like watching a TV show, or just lying down to go to bed. It would never be a pain that would distract me if I was focused on something else. Regardless, I don't think it is out of the realm of possibility that K could induce this kind of psychological uncertainty in a person who was already aware of the bladder dangers... so if it WAS psychological/neurological, then other avenues to explore might be some kind of hypnotherapy or psychotherapy, although honestly I'm not sure how you'd go about finding a practitioner with the aim to treat this, specifically.
Aside from that - and working again on the assumption that your pain or discomfort is physical in origin - there are a few other things you could try to heal yourself.
BPC-157 is probably the first thing I would try, I think, in your position - it's an interesting peptide which supposedly exhibits some kind of all-purpose healing properties for soft tissue, and there is some indication that it might be worth trying to heal bladder damage - although, it is worth noting, there is zero actual scientific research done on humans:
Study on rats with stress-induced urinary incontinence
Apparently successful anecdotal report of someone using it to treat prostatitis
Apparently successful anecdotal report of someone using it to treat pancreatitis
Actually, before you do that - I would try to quit drinking entirely. I see that you are taking a break from the other drugs, but cutting down on the drinking - I would actually do this the other way round. While the other drugs might aggravate the condition in an indirect way by causing you to forget to stay hydrated, or overexert yourself, or just the general, unspecific strain of taking a stimulant, there is nothing inherent about them that would cause bladder symptoms to get worse.
Alcohol, on the other hand, is just an inherently harmful drug, by most measures, and can actually cause cystitis on it's own (granted, usually only in very heavy drinkers) so there is a much higher chance of it aggravating an existing condition. I know that it is hard because alcohol is a social drug whereas the others, even if they may be social, are widely illegal and often stigmatised in many social circles, but if you are not dependent I would try first to quit drinking alcohol entirely for a few weeks and see if it makes a difference.
As ever, good luck, and I hope you get something useful from this.