Unless your finger is double-jointed, then the reason it's been collapsing is, in part, lack of muscle in that joint (i assume you're talking about the joint closest to the fingertip.
This joint often needs strengthening in thumb position, mine did.
You'll need to strengthen this joint over a period of weeks, and you can work on this away from the bass.
This exercise strengthens that specific joint.
FINGER PRESS-UPS:
1. Take your RH and give a thumbs up, straight and strong, as if to someone standing in front of you.
2. Now, coming in from the side, place the weak finger along the left side of the thumb, also pointing up, with the middle joint (not the closest-to-the-tip joint, not the closest-to-the-hand joint, the one in-between!) at the same height as the tip of the thumb.
3. Keeping the LH at this height, bend the weak finger so that the tip touches the top of the RH thumb (it's ok if the LH has to move a little away from the RH, but only just as much as is needed.
4. Apply lots of pressure down on the thumb from the LH finger. It should be bent like a letter 'n' rather than a collapsed, 'u' shape.
5. Now deliberately collapse the finger while under pressure. You should find that it 'pops' into the collapsed position.
6. Now deliberately 'uncollapse' the finger; it should pop back to the n shape.
7. Slow down the motion and the pressure so that you can feel the exact point at which the joint wants to 'pop'.
8. Slowly and with as much control as you can, move the joint beyond the point of popping in both directions. You might have to use a bit less pressure to achieve this, but the idea is to be using as much pressure as the finger can take.
9. Repeat as often as you can. With time this exercise strengthens the joint, and you can do it any time you're bored - like when someone is telling you a boring story, you can discretely be making your finger into a MONSTER.
enjoy.