Mr AG Curry hit the nail on the head; take your time.
Cut down on the hours and concentrate on relaxed practice. You'll be amazed at how quickly you progress.
Keep physically fit, take B complex vitamins and make sure you're not low in Iron or have early type 2 diabetes.
Potassium can help if cramping; Bananas, Tomatoes. But it is unlikely to be a medial issue.
The one activity which wrecks my bass playing is clipping the hedge with a petrol hedge trimmer but this is a right hand issue. Stamina comes gradually, and whenever you feel pain, stop.
I have engaged in intellectual pursuits where frantic cramming can be beneficial. I have tried this with Double Bass, but it doesn't seem to work like that.
What I have come to believe is that Double Bass technique and learning is a process, which involves much more than just your brain. No matter how many hours you practice beyond an optimum, you will not improve more quickly. But a relaxed few hours a day will bring you on much more quickly than you realise.
It's not a sprint but a marathon and wait for weeks rather than days to see real signs of progress.
You can also do a lot of this work away from the Bass, like memorising pieces and scales and fingerings.
Concentrate on tone, tuning, proper posture etc and, I'll say it one more time..RELAXATION.
Do the audition, but accept the level you are at, continue to practice consistently and by the time the next one comes around you'll nail it!
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