Overly submissive dog training

A little background. Jasmine is a a year or two old and a mix of what appears to be pit and dalmatian. She was found abandoned on the side of the road and is petrified by thunderstorms (obviously was left to fend for herself in this stormy florida weather). She is the sweetest dog i have ever come in to contact with. She cuddles like nobody's business and loves being close to us.

The problem I am having is trying to train her to stay. She will sit just fine and calm down when told to do so. However, I have recently been trying to get her to stay and she'll sit down and put her paw up (because sit is usually followed by shake) and then when i say stay and put my hand out she just kind of turns her body to the side looks at me and either walks away with her head down or rolls over on to her back in a submissive position. It's actually pretty amusing to me. How do I get her to stop being so submissive and realize she doesn't have to roll over on her back to please us?

Also I've done a bit of research and she has no issues with submissive urination or anything like that and she doesn't appear to have much separation anxiety. She just loves to be loved and does anything she can to please which unfortunately is getting in the way of basic training. She is fully house broken and HATES cages. She literally chewed her way out of her crate the first time we left her in it. bloody gums and all but she broke the damn thing haha. We leave her out and she is perfectly fine as long as the trash is put away if this info helps with your advice.

Sorry for the wall of text