Welcome.
It might just be a case of the fact that you're just not used to playing bass, and you just need to "stick with it".
You don't mention whether you play any other instrument, so I don't know if you're just not used to playing anything, or whether you've played something else that is a bit smaller than a bass.
Certainly, if you're not used to playing, doing some finger stretches before you play may help, and then play in short bursts, say 15 mins, have a rest for a bit, then do another 15 mins.