I wasted years losing at chess, until I learned this

The average club player does not need to memorize a ton of opening theory.  Most games leave the book within a few moves, often by move 5.  Deeper theory doesn't become important until a player reaches either 1800 or 1900.

Your goal in the opening should be to develop your pieces to active squares, castle, and don't create weaknesses in your position. It is likely that eventually your opponent will give you something that you can take advantage of.