I'm not a very good putter.
But after trying lots of different things to improve, what seems to be helping me the most is the repetitions everyone talks about here. There are a lot of factors that go into a putt and what I found is that when I stepped up to make my first putt of the round, or the practice before the round, I sometimes couldn't miss and sometimes couldn't toss one in the ocean off a pier. Why so good sometimes and so bad others?
For me, it was realizing a whole host of things I had to remember to do each time. I really started questioning everything, wondering if I needed to switch to a totally different style, or buy different discs or something. But then I see players who can drain everything inside the circle using similar techniques, so I figure it's not about looking for a magical putting stance or grip, it's just about working on it constantly.
I pitch putt and use "finger spring" to impart spin to the disc. So what I found was I have to remember some basics on every single putt: Keep my arm straight, get my "squared-up" stance just right, grip the disc at the right position (in terms of thinking of the disc like a clock, at 1:00, etc.), have my wrist in exactly the same place, etc., etc. If I start putting badly and wondering what the hell is going on, I really pay attention and in almost every case, I realize I've let one of those things slip a little bit. When I adjust, it gets better.
My current routine is I've set up a basket in my garage and I have four of the same mold of putter in it. Before I get in my car to go to work, I putt from the other end of the garage (about 18 feet) and force myself to make four putts in a row before I can leave. When I get home, same thing before I go inside.
This is not the only putting practice I do, but it ensures I do at least some every day. And it really forces me to focus, because sometimes I'm hungry but I can't go inside and eat until I make those four putts in a row! This also helps with the type of pressure you feel during a round, where you can't throw practice putts, you've got to walk up to your disc after throwing a drive or upshot and sink the putt on your first try. With this practice routine, I really try to make the first putt when I'm completely cold. If I hit the first four, I stop right there. I don't force myself to repeat it. It gives me a confidence boost I carry around for a while.
For what it's worth. Not sure this is helpful, except to share that I felt as confused and frustrated as you not too long ago. And I still do a lot of times! But it's getting better by me doing these things. Maybe they'll help you.