Yes if you had taken the pawn you would have opened the board up nicely. You are right not many casual players know en passant or forget about it, It's the newest of the chess laws & nobody knows exactly how old it is, quotes vary from 400-600 years.
Keep practicing you are a decent player. Hardly anyone beats me, I don't gamble on anything but my friends do if they see I'm about to play chess.
I will give you a few pointers...
When playing speed chess it's a handy trick to keep the point value of the pieces in your head eg. Always sacrifice a bishop or knight for a rook etc.
I always look to castle in the game as early as possible it makes you harder to beat.
From the outset look to counter attack rather than attack.
look at as many different options on the board as possible & try & read as many moves in front as you can with each option.
Defend your King with everything (Long live the king) Surround him with bodyguards & make an impenetrable defence for him.
Then invite the attack & pick them off as they falter.
The first to attack usually loses.
Always choose black if you get the choice it's an advantage (most casuals choose white)
If you are good at defending attacking takes care of itself,
Soak up the pressure block everything cover your pieces & frustrate your opponent.
protect every piece I get upset when I lose a pawn unless I'm making a sacrifice.
Good luck, keep playing, practice makes perfect.