Roached back when riding, leaning forward..help

I am writing in reply to start some more talks on the below thread that helped me so much with its replies (thread to old to reply to). I for some reason have started leaning forward, roaching my back and no matter how much I "will" my 'hiney' to get in the seat I cannot get it there. This doesn't happen all the time but I'm ready to go have a back specialist look at my back for problems since this is becoming quite embarrassing. I've rode since I was young but that doesn't mean you rode with correct seat. I"m ready to correct this. Not sure why I"m doing this, have a new horse, borrowed saddle but I seem to do the leaning forward in both saddles at a higher speed. I am taking lessons and have been told to ride bareback and I do try but please offer more suggestions.

Here's some of the ugly-back pics basically its when I get thrown off balance, I can see in the 2007 pic my stirrups aren't short enough to have a good leg under myself. And in the very last picture I was trying like the dickens to slow my very green, first time in arena girl down.






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I never used to do this, but about three months ago I picked up the new habit of roaching my back to help me sit the canter. It's really annoying and is an ugly habit. I don't like slamming down on my horses back (when im riding a choppy horse), so for some odd reason I started to roach my back to hold me to the saddle. Though it does work it's ugly to look at and I know i'll get marked down in shows for doing it.

Yes I know this means that im not using my seat properly, you don't have to tell me, haha.

Any tips to help me break the habit?