My goodness. What an awful thing to have to deal with!
Honestly, I would stop playing if I were you. If you can't yet get through a full match, you're just not ready.
Can you just play doubles? Can you change your playing style to finish points more quickly?
Since it is a flex league, another option would be to discuss beforehand with your opponents. Tell them that you can try to play, but you will retire if you feel you cannot continue and you have had to retire three times. If they still want to chance it, then they cannot complain if you have to reture.
-- Random Error Generator, Version 4.0
-- Master Moonballer