In some games, I was up on material, but then lost due to either time pressure or blunders. How should I avoid these type of pitfalls?
I see two things you can consider:
- trying to learn how to control your stress.
- playing many faster games to get used to play and think more quickly (if you often play games with 15 minutes per side, try to play games with 10 minutes per side).
Therefore you most probably won't often be in this situation anymore.
When you are up material, the simplest way to win is to trade material. Don't try to over complicate the game if unnecessary. Trade off the pieces and you will win the endgame -- easily.
This is a psychological issue. You have to change your thinking about what it means to be up material. Instead of thinking "I am winning," simply think, "I am up material, but what did I give up for that?"
Often to be up material you give up some kind of positional weakness, sometimes significant and that is probably why you are losing these games.
If you still see the very real danger you are in after being up in material you will play as hard as you should and not expect and easy win, and then you will find the wins. It's all a matter of not relaxing until the game ends.