#1
It's not the first time this happens to me. There was this fast riff at the end of a song, and my fingers memorized it. It's not like I didn't play the song for a month, I played it at least once a day for the last couple of days, and today, suddenly, my fingers forgot how to play the fast ending riff. It's like i have to go reeeeaaaallllyyyy slooooow to play it right and it's very annoying. Why the hell is this happening? Should I start practicing it again at slow speed or it's just that my fingers are tired now? (I've been practicing a solo for a while) Has this ever happened to anyone else?
#2
It sounds like you aren't practicing it enough. Getting it right once a day isn't enough - you need to be borderline obsessive on challenging stuff. Play it once, then again, and again, and again and so on. Also, slow it down and build the speed up slowly.

And then, practice it in context with the rest of the song.
Quote by EndTheRapture51
who pays five hundred fucking dollars for a burger
#3
while practicing a fast riff, if it's short, is it recommended to practice the whole thing or practice the small parts individually? (I've never had a teacher)
#4
Each part induvidually, then merge them.
Quote by EndTheRapture51
who pays five hundred fucking dollars for a burger
#5
Quote by Yekex
while practicing a fast riff, if it's short, is it recommended to practice the whole thing or practice the small parts individually? (I've never had a teacher)

It's recommended to practise sections you have problems with, so it's good to reduce the riff to the bare minimum to practise the hard part. Don't try to play a riff fast before completely memorizing it slowly. Don't just go for full speed and hope to hit the right frets.
E:-6
B:-0
G:-5
D:-6
A:-0
E:-3
#6
If you're forgetting parts a day or two after learning them, you haven't gotten the muscle memory down. I've got riffs that I learned a year or more ago and can still play passably well at the drop of a hat. Muscle memory is literally why some things are like riding a bike. It just becomes natural to make those movements.