OMG I am having the EXACT SAME PROBLEM!! 4 year old mare. Started last year. Great until the temperatures dropped. Then sucked back, refusing forward, rushing and tense when I demand forward. Sent her to a trainer for a month...same problems. Vets can't find anything wrong. Weather warms...totally different horse! Forward, soft, round and happy. We had a great summer and fall. Temperatures dropped 2 weeks ago and she revered alllllll the way back to her tense, sucked back pissed off behavior. I've given her 2 weeks off just to try to figure this out! She's turned out in a big pasture with little grass (air fern) and stalled at night. She was always out 24/7 until November. She is blanketed, ridden in the same indoor, no change in weight or feed. I'm having my saddle fitter out to verify everything is OK. Doesn't seem off at all and just had her teeth done. Soooooooo wierd as I could have written the original post myself! After her vacay ends if she is still pissy and stuck I'll be doing a standard elimination trial. Already tried all my saddles, changed bits, tried getting off her back and long walking warmups. She is still the same. Ears pinned, tense and refusing forward. Utterly bizarre! And she's SUCH a willing and happy, silly girl it has me quite perplexed. The only thing I could find was the temperature as far as a consistent variable. So interested to read all the posts and see more!! OP I feel your pain!!
"We don't stop playing because we grow old; we grow old because we stop playing."