Life In 19x19

jts wrote: ...... , B throws away several opportunities to crush me. I was really hoping some people would have ideas, either strategic or tactical, about how to not be in the position I'm in at b139 at all.

Well, I can't hope to be much help to you as you are much stronger but maybe someone else will react to my attempt.
Second game: To me it seems that after you realized you made a suboptimal move you raise the stakes by making even worse overplays. This way he can avoid to take the big risks that you take.
20 tries to repair for 16. C3 would have be a good preparation for that invasion. It attacks and defends. Q18 is the same idea.
58 tries to repair for 20. It creates a third weak group.
At 86 you are desperate so you stop counting liberties. Counting would have made you play 96@P12 to save the 5 stones. No big deal but still.
At 114 you have a nice move at M5. Probably not enough to gain the lead but enough to continue the game. After you missed it you were at his mercy.