When I try to do a roll, my fulcrum clamps up so there's no space between the fingers... I can't get it feel/look like a correct holding technique at all... yet my right hand is perfect! Why... ugh. I wish I was ambidextrous, it's so annoying!
This is probably obvious, but you must realise that your fulcrum doesn't clamp up by itself --
you're clamping up your fulcrum, and to fix the issue,
you need to stop doing that! There's no magic involved, and ambidexterity has little to do with it.
Like PfoG said, you just have to take control of your muscles. Play slowly with your left hand, and
focus on keeping the grip relaxed. Let the stick bounce and don't squeeze the stick any more than is necessary.
Focus on your left hand for at least ten minutes every day, and get a
feel for the relaxed muscle action and stick bounce.
I know I spent a few hours every day for a couple of weeks before I had the free strokes and double strokes down on a basic level. Once you have the basic technique down, start working on
Stick Control and rudiments to further integrate the stroke to your playing. Play grooves and fills, and
focus on your left hand.