It's OK to put a piece on hold, for whatever reason.
I find it interesting that you can play it at about 50% speed. My thought would be to quit focusing upon it so much but fairly often, come back and play through it at 50% speed knowing what it is, but just come back and do that often. If you keep doing that, you may find that it'll become easier to take it to maybe 60%. And it's OK to leave it there for a while. Etc. It may take a year or two before it "clicks" and becomes something that your fingers remember, not just your brain, and suddenly, it's as though you're not making it happen, it's just flowing out of you.
I have occasionally had tunes that I tried that just did not work that for whatever reason, I revisited some time later and all of the sudden, they clicked. No idea why.
Maybe some players are different and can remember things exactly for long periods of time. I'm someone who forgets. If I don't actually play a tune for several months, it's sometimes a little iffy as to whether I'll remember it right. I seldom play with any kind of music other than maybe a lead sheet but the similarity from one song to another can be close enough that without concerted effort I could very well start out playing one song and end on a completely different song. Doesn't happen when I play through them fairly often. Ones I haven't played in a while, though... dangerous for me, at least in any situation that matters. Interestingly enough, at least to me, I have had instances when I've had to play for a singer and a song I couldn't exactly remember, but as they sung it, it just "came out". I have no idea how that works but have experienced it numerous times.
This music thing is fascinating, and goes way deeper than the intellectual playing of notes in organized sequence.