1. Set your bass up with a low action - allen keys are cheap. It's fun setting up your instrument to your own taste. There's heaps of vids around to show you how.
2.
Get a decent teacher. If you
absolutely cant find one -
Online Bass Guitar Lessons | Scott's Bass Lessons - Do a beginner course on here, or check out his youtube channel.
3. Find some 4 finger warmups that cover the whole neck - get a few ones to work on. - on the net or youtube Remember *thumb behind the neck, using all 4 fingers curved over - and using the tips of your fingers - practice them every day.
4. Learn all 7 modes , 3 minor scales, the Blues scale, major/minor pentatonics, whole tone, the 2 Diminished half/whole and whole/half scales - learn the arpeggios that fit over each of these scales - Learn the One octave patterns - then expand. Practicing these will also be part of your warmup - and it's a good way to find patterns you like, and help you learn shapes to play with.
5. If your hands start to cramp up give them a good shake and move on to something else, If it's really bad have a break for a while. Your practice should go: *Warmup, *scales and Arpeggios, *riffs and songs... If you do this everyday your stamina
will get better.
6. Skip what that guy said about the hand strength gizmo. Don't play through pain, just move onto something else. And either come back to it later or tomorrow.
7. Make sure you are
actually healthy, and getting all your vitamins - You will definitely need all your B vitamins, Potassium, iron, magnesium and calcium - the last 2 being very important for tendons and muscles. and of course - water. *If you drink or smoke a lot this could be affecting you stamina and vitamin intake.
I am guessing you don't have a teacher or he/she would have told you all this stuff... And you wouldn't be asking us

. And I am also guessing you don't have much of a practice regime - because a good teacher would MAKE you practice this stuff every day - And you would have a fair bit more stamina.
Cheers, I hope this helps.