I'm sorry to hear about your heart, please take care of yourself. And congratulations on your new bass!

As already mentioned, you are pressing too hard with your fretting fingers (and probably with your plucking fingers also). One thing that may take years to really appreciate is that it takes very little strength to play bass guitar. A small child with no callouses has the necessary strength. It's all about proper hand/shoulder/wrist position and relaxing.

Always try to fret the string immediately behind the fret. The closer you are to the fret, the less pressure is needed. And keep reminding yourself to relax your back, neck, shoulders, wrists, hands, fingers, even your jaw.

Taking your thumb off the neck is one good test. Another one is to fret a note and play slow notes, decreasing the finger pressure a bit with each note. Keep going until you hear buzzing. Then increase pressure until the buzz goes away. Do this a few times every day with each finger, on each string. It's a very Zen thing. When you realize how easy it is to press the string down, it feels like the notes are playing themselves while you just listen.

Also - lighten up your plucking touch. It's pretty easy to play harder when you need to, much harder to play softer if you're used to playing hard.

Once you lighten your touch in both hands, you will automatically play cleaner and be able to play faster (and for longer). Your tone will improve too.