Some tips on time management? I believe I would've won this game had I had more time.

Time control was 15+15.
A classical game against an opponent whose classical rating was 2316 (2550 rapid!).
In the middlegame he made a mistake and I grabbed 2 pawns and I got into a won rook endgame. I had 2 minutes to find the winning idea and I later blundered allowing my opponent to enter a drawn position, I tried some tricks but my opponent played precisely and the game ended in a draw.

According to sixth world champion, you should use at max 1/3 of your total time in opening.Then a move can require a lot of time and you must do the following ones quicker, it is the normal use of time.