Let's say I've used wild shape to assume the form of a mammoth that has 126 hit points. I take 26 points of damage, putting me at 100 hit points. If I use my next turn to take a bonus action to exit wild shape then an action to use wild shape to assume the form of a mammoth again, do I now have 126 hit points or 100 hit points?
You get the full hit points of your new form whenever you use wild shape
When you enter wild shape, you assume the form of a creature and get their full hit points regardless of your previous form's or your previous wild shape form's hit points.
When you transform, you assume the beast's Hit Points and Hit Dice.
It says nothing about ever receiving partial HP for any reason.
Each time you transform you are effectively creating a new form for yourself (not reusing the old mammoth form for example).