I swtiched to the DB a few years ago.
Everything you say in this thread, I went through it, too.
The pain, the 5 minutes and have to stop, the lack of confidence, the giving up.
But I stuck to it, and know I can play all the time I want.
I'm not really sure when the shift happened, but it did.
I think it was when I joined the Jazz group and started to play more. And at the rehearsals I just kept going.
I know I tweaked my left hand technique, I just can't explain what it was.
I press with the thumb. Everybody does.
We played a tune this season, Seven Steps to Heaven, and I couldn't play through the track for more than a couple of minutes. It was too fast and both my hands got fatigued really quickly.
So, I played it hundreds of times at 50% speed. Then at 75%.
And then I just could do it, at full speed and even faster. And then, I could play a full set. And then more than that.
What I know I did:
Played every day.
Didn't let the pain affect me. When it hurted, I played something else. But kept playing.
When I got fatigued playing Miss Jones, or Seven Steps (lot's of half position and 1st position), I played Footprints, or Mr. Lucky.
But I didn't stop. After some time, I went to a work medicine appointment, mandatory for work. I was asked to press a ball, and they measured the psi. My left hand was stronger than my right hand. And I'm the most right handed person in the world. The doctor even asked me if I was a lefty.
Make sure you don't injure yourself, sure. But the change will come.