I have zero mental resilience

I cannot finish a match once I am ahead.

Just lost a match where I won the first set 6-1. Then I was down 0-2 in the second and lost the second set 6-1 where I gave up. The super tiebreaker I focused hard and was up 8-0 and lost 11-9.

Extremely similar scenarios have happened numerous times. If I win the first set I can guarantee with 100% certainty I will lose the second set. Perhaps ADHD is a contributing factor.

I cannot even remember the second set. Physically I am in good shape, but my mental is incredibly crap. In fact, it gets worse with every match I play.

Once I am ahead, I just give up because I consider the match won. I understand I can't do this, but how? How do I train and improve mental resilience?