I admit I didn't go through all of the responses so apologies if this has been suggested. First desensitize to the mounting block. Use a round pen or other small enclosure..what you want to do is limit how far away the horse can get form the "scary" object. Put the block in the middle and just let the horse roam around. Horses are naturally curious and eventually he will walk up to it, sniff, paw, snort....
Do this again, or at the same time, with a saddle on their back..get them used to the saddle and the mounting block. Obviously if the horse is prone to rolling, use old tack :) If they have already seen a block and have been tacked up several times this part won't take long.
Someone else mentioned the next step, the old, walk up to the block, stand, get a rub and trrewat, move two steps back/sideways, you reposition, rub, treat, move the block, reposition them and get them to stand still, rub and treat..put one foot on, if they stand still, major praise like they just won the Derby, treat..you get the idea.
My horse was like this. He would only stand for the block if it was in the corner. The first time I worked with him in the middle of the ring it took 90 minutes and one heck of a lot of treats and he finally stood still. We still haven't beat it 100% but he is getting better. I did use Clinton Anderson's technique of tight circles and backing but due to a possible (even the vet isn't sure) front suspensory strain, I hesitate now to use that particualr technique, at least on mine, since neither the vet nor I can figure out what caused it.
As for the riding, reintroduce the mare to weight on her back. Lean on her back, not actually mounting. While laying there, rub all over, praise her for being still. Repeat as needed. The idea is to get her used to the weight and the activity and get her to realize she doesn't have to "run" when weight is on her back...it sounds like she may still be carrying around race track memories where weight on her back meant "go really fast."
The important part is to remain safe and if necessary, locate a trainer who can help you retrain her.