shants

if the bar doesn't fit into stem:

1. check to make sure that the hole in the stem (the clamp) is big enough for the bars. check the diameters of each. technical note: your nitto stem most likely has a 25.4mm clamp. the protapers come in 25.4mm and 31.8mm varieties. naturally, the 31.8mm bar will not fit in the 25.4 stem. from looking at pictures of the 31.8mm protaper bars, it looks like you'd be able to get the stem up to those bends. that's where the diameter of the bar seems to drastically increase.

2. if the bars are indeed meant to interface with the stem, then check to see if the stem clamp looks crimped or otherwise malformed. sometimes idiots will tighten down a stem clamp when there is no bar inside, which will cause the hole to become smaller. you can remedy this as suggested above -- pry it open.

i suspect that once you evaluate the things in steps one and two, you'll figure out what the problem is. sometimes it can be a little tricky to get a stem onto certain types of drop bars, but the agony that you describe is simply not going to happen with risers unless something is afoot.