How did you improve your amateur mistakes in chess?

Your first mistake isn't how you play the stonewall attack but THAT you play the stonewall attack. Play something with 1.e4. The stonewall attack leads to closed positions that aren't very good for your development as a player. Learn opening principles instead of systems so you'll know what to do when your opponent goes out of book.

Slow down and think about your moves and your opponent's responses. One move mistakes come from not paying attention.

I have no idea what you mean by "correct symmetrical moves". Most really good players avoid symmetrical positions at all costs.

I don't see how you can blame your memory when you're talking about one move mistakes. If you can't remember the position look at the board.

You're also not really using the word "erroneous" correctly.

Before every move, think: what can my opponent do to me? S/he's just made a move - can I see why it was made?

A common fault amongst relative beginners is to concentrate on attack without first considering whether a defence is necessary.

30 is young for a chess player - you have your best years ahead of you: theoretically your mid to late thirties.

Play fewer blitz or rapidplay games. Give yourself time to consider more possibilities for every move.

And join a chess club - the best move you can make!

Practice

If you play in tournaments then you will have written down the moves as required. In that case go through the game and find out which moves caused you to lose the game.

I only ever learnt the rudiments - but with so many (almost countless) alternatives of defence/attack - SETTLED

for just playing the game for FUN. WINDING-up better skilled players with unexpected moves - feints and distractionary manouvres which puzzled them - until they realised they could wipe me off the board ; ))

You should follow the excellent advice from Aquarius35.