I do 15/10 most of the time.
Stuck at the same rating for a long time
What is the difference between beginner and intermediate player? Intermediate players know more tactics, mating patterns, opening and all other ideas about the game.
So maybe to break 1500 you need to gain more ideas in your game. You can watch all the video and read all the books but you won't be able to use it if you're playing blitz. You need to slow down and think about all the ideas even in the first three moves.
Why did your opponent moved the pieces? Did you see the best idea? What is your idea when you move your pieces. Is the idea a few moves down or a combination of pieces...
Chess is a thinking game so get to thinking
Switch to 15|10.
5|5 is fun but no way to progress.
"He who analyses blitz is stupid." - Nezhmetdinov