How to improve in 10 min rapid from ~ 1500
@1
"I blunder a lot" ++ Always check your intended move is no blunder before you play it.
Invest a few seconds to double check before every move.
You may occasionally lose a game on time, but you will save many losses by blunders.
"choosing Ruy Lopez for my repertoire was a bad" ++ It is good.
"how to improve my rating on rapid?" ++ Whenever you lose a game, stop playing and analyse it thoroughly so as to learn from your mistakes.