You definitely need(ed) to do something there, and I suspect the foam was not the best idea. The bottom of a tub needs a solid foundation, and that foam is easy to crush.
I would stop spraying foam and find a way to get something solid under your tub before its bottom cracks through altogether. That happened to me a couple of years ago, and I ended up cutting out the bad part and glassing in a new bottom. The work you need to do (or to have done) might be difficult, but the alternative is the complete replacement of a bottomless tub.