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You say the transition from games at 15+10 to blitz sees you going from rarely losing on time to almost always losing on time.
Can I ask if the blitz is still with increment eg like 5+5 or some such?
When I switch from say bullet to 5+5 to 8+5 and such it always takes me several games to get into the rhythm of the new time ...
There are plenty openings you could use in blitz, but, i think the slow solid and positional openings need more thinking, thus you could lose. I would suggest playing like Mikhail Tal, make sacrifices without calculating the move too much, and try to complicate the position for your opponent. I have used this in many of my games,i complicate the position, ...
What do you call blitz?
When I played it was zero seconds per move.
Now it seems to be variable like SD1 to SD5.
At your rating it really does not matter what you do.
For sure you will not ever improve if you play blitz.
It is more likely to reduce your actual rating a LOT.
What you need to do is improve your skills.
Then practice at playing at exactly ten ...
I can understand your situation. Because I am also a classical player, rated 2109 on lichess; but performing poorly on the blitz (1712) and bullet (1741). Moreover, there are more than 30 games, where I lost on time though the positions were in my favour. So, I think, I can give you some tips to improve your ratings by at least +300 points.