Practice long rallies in matches on purpose.
That is the best way to build up "shot tolerance" and not become anxious to end the point after just a few exchanges.
(This is NOT the way you will want to play forever - in the future you are going to want to exploit short balls by hitting approach shots and coming in.
You are going to want to hit your first serve for aces.
You are going to want to "go for" flat put-aways in to the open court.)
But to build shot tolerance you may want to play a few "pusher" on purpose, and try to outlast him with back court rallies using safe shots.
While this may not have any immediate appeal for you, it may be just the thing you need both to get over your low shot tolerance, and to build up your stamina (may be better for you running on the court than in a gym.)