Intervals. Also, if you have to ride over 19mph to go over 19 mph.

You must have a program. In other words, you have to learn to ride slow before you ride faster.
Don't just go out and do a PR each day.

walk the bike on mondays
tempo on tuesdays (19 mph)
intervals on wednesdays
tempo on thursdays
walk the bike on fridays
balls out, as fast as you can go for your sat or sun rides.
I mean, do whatever you need to do to hang on.