Black's was able to fork your pawns with the strong 14... Qb6 because you had a bunch of undefended pawns. In general, try and protect your pieces even before they are attacked, to make attacks more easy to defend against. This is called over-protection, or prophylaxis. Also, taking more time to think about each move will help you avoid blunders such as 20. a3. Hope this helped!
What did I DO WRONG?
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I think that mistake came with 14. Rae1. Better was 14. Rad1 or Na4 with idea of Nc5, but knight at side of the board is not good and c5 isn't strong enough. Also 16. Qe3 isn't good because Black already has one pawn advantage and another pawn is unprotected - better could have been 16. Bd2 and if 16... Bxc2 17. Qxe6+ regaining some material back as well as isolating d pawn which is advanced a little more than it should be... That is opinion of amateur :o)
Here is a game I played a while ago.