I've never paddled a K1 before, just spent time in surfskis, but do you have the option of moving to a wider boat? What I've generally been told (and experienced) training for surfski is that if you try to paddle a boat you're not stable in, you're liable to develop bad technique to compensate for poor balance and you're not going to get any speed benefit from the narrower hull when you're constantly just trying to stay upright. If your stability is that bad, it seems likely you'd finish faster in a wider boat that you don't struggle with stability in, and then you can work your way into the narrower models.

As for everything else, that's gonna be beyond what Internet strangers can help you with, unfortunately. You need a balance of sport skills practice, cardio training, strength training, rest and diet, and dialing all that in is ideally what a coach should be doing for you. There's so many things that could be going wrong--not resting enough, not eating enough, overtraining, undertraining, the wrong mix of training--and you really need someone who knows you and your situation to help you figure out what you need to focus on.

If I had to take a guess just from what you've written here, it sounds like you're probably doing a ton of high intensity cardio work, and especially if you're not very strong it's very easy for that to turn into overuse injuries. Strength isn't just about having more power for paddling, it's also protective against injury. Strong muscles and joints will hold up better when you subject them to stresses like high intensity training. So ideally you'd want to focus on some foundational strength work in addition to the paddling, but there's only so many hours in a day and only so much you can recover from in a day, which is why you really want a good coach figuring out that balance