I would have been in trouble even without looking at the clock

Even though I've only started playing a lot of chess in the past month, it is already clear that I am terrible at the endgame. The game I just played exemplifies this. I think the time limit was... 15 minutes? Plus 5 seconds after every move? This is important, because around the 34th move, I realized I had spent so much time thinking about my endgame that I only had 2.8 seconds left on the clock. I did eventually lose on time. However, I am sure that I went wrong before that. Please help me figure out where.

If you want to jump ahead to the endgame, I think it starts with my 20th move.

It was a good game up until you started running into time trouble, lots of mistakes by both sides though. You missed 19. Qxd7#

Oh my... I can't believe I missed that. I usually nail that sort of move.