Maybe the opening isn't my weakness. I usually play the first 7 or 8 moves pretty accurately (they're in an opening book and have a name although I have no idea what it is at the time). In this game I achieve a winning position. Then I make a tactical blunder, but white blunders back, and I maintain a winning position. Then I make a positional blunder followed by a tactical blunder and lose.
1. Notice open files. They're probably important.
2. If you miss #1, and your opponent takes the open file, don't let the enemy rooks on your 7th rank.
3. If you miss #2, it's important to calculate and defend accurately.
4. While attempting #3, remember that the king is worth more than a pawn.
I have an illusion that I'm smarter than this. Unfortunately I continue to produce evidence to the contrary.
Okay, enough self loathing. The real lessons:
1. Always consider all forcing moves (yours and his). Do not attempt a combination without considering these moves, especially checks. They can interrupt and change everything.
2. Do not become lazy or complacent after your opponent makes several weak moves in a row. You must still calculate and find the best moves for both sides. If you start assuming every move he plays is weak, you'll stop analyzing, and your moves will become weak at best, and you might blunder away the game at worst.
3. Pay attention to open files even if they are currently blocked by pieces and look like dead ends. If the pieces move with tempo, the file can quickly become a highway into your position.
4. Don't get angry at yourself when you make mistakes. Everyone makes mistakes. The difference between losers and champions is that champions learn from their mistakes and become better because of them. Every mistake is an opportunity to learn something and improve.