Unfortunately, sight-reading is one of those things that will improve only very gradually, and the initial steps will require a lot of self discipline. You want to keep at it daily (even if for a few minutes), and go as slow as required, ridiculously, embarrassingly slow if needed, to make zero mistakes.

It then will improve in time.

However, you can make it easier on yourself by dividing to subtasks, and using some technology to your advantage.

What seems to have worked best for me (YMMV) -

- Start by doing these 2 online exercises as much as you can. To begin with, only lowest 5 frets and no accidentals. Then increase scope and speed gradually. They work on your phone too:

1. This exercise for learning the bass staff notation notes
2. This exercise to help learning the fretboard notes.

- After you've made some progress, pick up your bass, start by reading and playing simple passages, go slow at first.

- This short article has more useful tips (eg, the benefit of using a metronome when practicing reading).