You inquired how you can be a better strategist. That can be a difficult question to answer since the specifics of each game influence the strategy. What I can offer is perhaps a broad perspective regarding 10 ball.
There comes a key moment in every game where you will be in a position, based on your skill level, to run out. Maybe it's at the 5 ball, perhaps it's at the 3 ball. It all depends on how the balls are lying and your ability to assess your talent.
Whoever is first to be in position on their key ball will likely win. Your objective is to analyze the table and plan your attack so that you get first crack at the run out when you get to the key ball. As the balls get moved around the table, you may have to change your key ball and also your strategy.
This means that you have to calculate when and where you may have to play a safe to ensure you get first crack at your key ball. You may even find yourself playing a calculated safe on your key ball in order to get ball-in-hand on your key ball, to start the run.
Your weaker opponent is probably beating you because you are giving him more opportunities to win when there are few balls left on the table instead of planning your strategy to be first in position for the run out from your key ball.
Many APA 5's can run 4-5 balls on an open table so the game strategy, at this skill level, is really executed during the first 3-5 balls. This is where you should plan your strategy and safety play so you have first shot at the run out from your key ball.
If it's a really ugly table, your key ball may be the 7. Regardless of the key ball, make sure you are first to have a clean shot at it and you will win those games more often than not.