I have a 12 month old GSD female that is a little on the soft side. I started training her when she was about 4-5 months (had her since 2 months).. My methods were more compulsion based and now I realize I have messed up. She is fairly obedient considering the off and on attitude I took with her training but she does not enjoy working(ears pulled backed at times, will show belly when platz command is given every so often). My schedule has opened up a bit and I would like begin on the right path. Is it possible to just start over? I am not really concerned about titles but I would like to at least attempt a BH and I really just want her to enjoy it. I have ordered some of the leerburg videos - drive and focus and competition heeling - but have not received them yet. Will these videos help me get on the right path or is there something I need to work on before I start the videos? Any guidance will be great.

Both of the videos you have mentioned will help lead you to repairing the relationship problems you have created with the dog. All is not lost, it is best you realized this now rather than never. However, when you have a foundation based in compulsion the dog will revert back to that in times of stress(for the dog OR the handler) so you may(or may not) be able to get to a highly competitive level with the dog. Just to learn how to enjoy your dog and teaching YOU how to motivate her for the BH sounds like a good start!

I would start over by making it fun for her. No corrections at all until well down the road. I would just play with her, ball, etc. Have fun with her and try to build a good bond. Then start throwing commands here and there during play and as soon as she does it give the ball or whatever reward you use, and act so happy that she did it she will think it's the second coming.
This would be a good start and then incorporate the videos in down the line. I dont know how much damage was done on your relationship with her but it seems that you should focus on the relationship with her first.

What about correction for bad behavior? Should I stop using "no" for a while? If so is there a way to stop bad behavior using possitive reinforcement?

Depends on what you consider 'bad' behavior. Killing the cat, that's BAD in my book and I'd be on my dog in a second. But sitting too slow? Not bad, just not right.

I know I have a sound I frequently use to mean 'no' but in a lesser manner, like an 'uh uh' (try to say that?) that's more of a warning than a 'you are a horrible bad dog and mommy is coming to get you' type noise. In the 'timing is everything' world of training, this warning noise works best if I catch my dogs right BEFORE they do the 'wrong' thing (like as the dog is sniffing the chicken I left on the counter and BEFORE the dog grabs it and runs). I found I had been filling my dog with a day full of 'no's' (very negative and uninspiring for a dog to learn) when instead I could frequently use my 'uh uh's' and/or redirect to something that I did like.

Lot's of play for a reward (dog 'sits', we play tug), treats, lots of praise thru out a training session. I try to set my young dogs up to succeed rather than fail, and if I can manage a situation rather than 'correct' (hide the trash can in a cabinet rather than yell at the dog for getting into it all the time) it can help.

Most of the time I have found that my pup isn't disobeying because she's evil and is deliberatly not listening. It's because she is not really sure what I'm asking her because I haven't trained it properly, or consistantly, or so she's getting my point. So it's not that she's wrong, more that I'm not doing it right so she understands. And then for me to yell at her cause I'm frustrated and angry (and she's confused) does not usually (at all?) help. So I keep sessions VERY short now. Use a toy as a reward and motivator. If I keep doing the same thing, and she is not getting it, then now I know I AM THE PROBLEM! And I have to stop the training and figure out another method to have her understand.

Always end training with the dog succeeding (even if it's a basic 'sit' then the play for reward). And if you are making it fun, you end BEFORE your dog wants to. Always leave them wanting more!

I was referring to no corrections during obedience right now. That is what seemed to be the main problem and where she got all her bad feelings from according to your post. If by bad behaviour you mean house manners etc...I would say you can still correct those if she knows them. Just correct calmly and be fair and dont be a nag corrector. You can use other methods on some bad behaviors like redirectiong to a good behavior and reward that good behavior...
During play/obedience sessions dont use any corrections for now though.