The best way to improve at blitz is to become a stronger OTB player.
Of course, if you only want to play blitz, you can try to identify specific weaknesses to work on. Here is a method I have used to do this.
Go through a large number of your losses—say, 100 or so. A smaller number will not work so well because clear patterns won’t become apparent.
For each loss, identify the reason for the loss: mouse slip, calculational error, lack of understanding of the opening, endgame blunder, time pressure, etc. Players of different strengths will likely identify different types of reasons.
with 100 losses analyzed, you should be able to identify several things to work on.
Bg the way, I have done this with my OTB games. It helped pinpoint certain openings and types of pawn structures I had trouble with.
j also used this when I was playing in duplicate bridge tournaments.
My blitz games make some sense