I'm about the same measurements ... 29/30" and I've always worn the XS tritop (I do both the black and the white, although, yes, the black ones tend to be smaller/more stiff).
I know that the getting-into-the-binder process is individual for a lot of folks, but this is how I do mine: I get mine over my head--I'll put one arm in and hook all of the fabric up to the armpit on the one side over my upper arm, then stick the other arm in--hooking it over the shoulder. Then you can use your arms to stretch the binder a bit more than it would otherwise do on it's own. I then grab the back of the binder and stretch it over my head, shunt the binder down the shoulders a bit by shrugging into it, then I'm able to wriggle it the rest of the way by grabbing the sides or back with my hands.
I also recall that doing it this way you may initially get some rubbing from the edge of the binder as it moves back and forth at the armpit area.
If you do manage to get it on and off, after about two weeks use the fabric becomes a lot less stiff, and more stretchy, which makes it easier to use, however, if you really, absolutely cannot get it on/hurts to do so I'd definitely try to exchange it for a larger size.
