Failed to capitalize on a better position

I had a time and space advantage in the middlegame, but foolishly traded into an endgame, thinking I would have a superior endgame position. I didn't. I would like to know how I could have capitalized on my position before I swapped pieces

, and whether there was something different I should have done in the endgame.

Not only you couldn't win the endgame but you could lose it(30...Rab7 wins a pawn because if 31.Rb1 then 31...Rxc5.

   The exchange of pieces on move 17 seems that is throwing away most of your advantage.

      After you exchanged the knight you could keep the queens in the game and le him exchange on d6 if he wants.You could continue with something like 18.c5 or 18.f4.

    Another critical exchange was the one of light squared bishops.Your last practical chance was to play 21.Bf3 because Black's bishop doesn't do much on a6 and you could press on k-side taking advantage of your piece superiority there.Of course he has q-side counterplay and you also have to prevent his bishop from reaching d5(or k-side) but if you wanted to push for win there was no other way.With only rooks in the game it's a  draw unless one of the 2 does a serious mistake.

17. Bd2 or Bd3. One reason for this is because two of your big advantages are space and activity.

If you trade pieces off, your opponent doesn't mind less space, because he has no pieces that are restricted by it.

If you trade your active pieces for his bad ones, you will lose some of your advantage. Whenever you trade, think to yourself "which piece is better?"

I think its fine if the heavy pieces get traded if you are going to have a lasting bind, but the way it went down he had time to trade off the light square bishop.  So I see two additional culprit moves that address the time issue:  16. g4 addresses a problem you don't currently have...by contrast, bf3 seems to force bb7.  If that is the case, you then follow rd2.  Now, if black plays h6, you can trade, triple your heavy pieces on the file and black must play bc8.  Who cares if he trades queens, it will take him lots of time to untangle and get and life for that bishop.  Even if you don't go that route, why did you trade queens yourself?  Rd2, and if trades, you'll double rooks and tie his bishop down unless he rushes it to a6, in which case double and play c4 and his pieces are just so passive.

Thank you to all for your analysis. It's always surprising to a low-level player like me when a seemingly normal developing move such as Bg5 turns out to be subpar.