Teaching My 7 Year Over Nervous Dog To Walk On A Lead.?

okay, i have quite a big problem.
my dog is 7, she is a collie X greyhound. she loves being out running and is so happy. but when she is home she always looks so sad and has her tail between her legs even when she comes to us for fuss (which is very rare). she has always been like this so we believe it is bred in her.
we are not able to take her for a run everyday because the place gets so muddy and wet. we would love to be able to take her for a walk but once on th lead she pulls and yelps and whines all the time. so we cannot take her. and we cant then take the other dogs either because we can leave her home alone because she just whines all the time.
we tried taking her to dog training classes but because she was so scared she did everything straight away, but she is nothing like that at home. she is extremely nervous and runs and hides from everyone except us.
any ideas?
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