If your goal is play upright, rather than quitting, perhaps stop over playing and focus on using good technique. If you quit, it's likely you will lose any conditioning and skill you have developed. So when your are ready to start learning the upright again, you will be back at the beginning.
To me studying jazz is more about theory and musical ideas...I don't see it relating to a specific instrument. When I was in school I learned new theory ideas in class and/or from a book. Next I applied those ideas to piano. The last thing I did was translate what I had learned to bass. After that I tried using the concepts in some sort of ensemble.
If you have more facility and endurance on bass guitar, I can see your teachers logic. That does not mean you shouldn't also put in some work on Simandl exercises to bring along your technique on upright. Just don't overdo it.