I was making notes to myself as I read down this thread, and when I got to the end to Phil's post, it actually was the same as the first note I made to myself, LOL.
revjames, veery's comments about not playing dead on the beat are excellent. However, I think first I'd check to make sure that it's not a physical coordination conflict problem (as Phil noted above) rather than a matter of a conflict in concept, first. Just to check this out, I'd give yourself a little test.
Take a metronome, set it at a reasonable tempo, and play the same pitch in quarter notes (crochet?) exactly on every other downbeat. Can you do that?
If you can't, (and assuming you can on your bass), then I would say it's a physical problem and that your embouchure, air stream, tongue, etc are not reacting with the kind of control you need them to react with. I don't think this thread is the place for going into the kind of detail needed to coach you on all of the various exercises and concepts you will need for that. Perhaps some use of the search function or starting another thread?
If you can play on the alternate beats, then I'd expand that exercise to consecutive quarter notes.
If that works, do it with eighth notes.
If that works, go to one of the tunes you are trying to learn and play with a metronome and play the first and third note of each measure, as you play through the first several bars of the tune, emphasising the downbeats, and making sure that you are precisely with the metronome.
If that works, do it with every beat in the measure.
Now play the tune with the metronome, accenting each downbeat. It's not musical, but that's not the purpose.
Once you can play the tune with the metronome, transfer to BIAB and play the tune, again emphasising every downbeat.
when you have that under control, you can gradually loosen the reigns, but you always have to know where the beat is and be able to return to it.
I also agree with VetPsycWars comments about being so trapped in the music, you loose awareness of what's going on around you, musically. the music. Our awareness and ears vary, of course, but I've always found it helpful to listen to the ride cymbal for orientation to the beat. Sure, the bass lays it down, but it's low acoustic doesn't always make it through the group sound as well as a high-pitched instrument, at least for me.
Ouch. I just read this: "I am always finishing bars too early/late..." That's a disaster!
Well, there are plenty of suggestions above. There are no shortcuts. frankly, if you are finishin late/early by a matter or entire bars, maybe you should re-think doing that upcoming gig, or at least doing it differently.