@Honeeecombs I totally feel with you with the painful spasms. My urge IC is working a little similar - if an urge comes up, it's getting stronger and stronger within seconds. Sometimes, if I try to hold in, after a few seconds the urge fades out and everything is fine. But more often the urge grows up to getting quite painful up to that point I can't hold in any more.
What's your guess I started do do after a few weeks happening this around 15 times a day (+ the nights)? Right, I stopped trying to hold in hoping that the urge fades out. As soon as the urge grows up I release in order to avoid those painful cramps, which works very well! If I'm really near a toilet, I go there, otherwise I simply use my diapers. I've got used to this, I can live my regular life as it was before becoming IC, nobody else can see, I have no pain - so what?
I'm aware that your situation - being much younger than me - is a different one. But with the choice being either more continent with pain or being IC without pain it was clear what I wanted...