You do with your car and money what you wish, but if you're truly regretting the Mustang and fully intend to trade or sell it in the short term to get back on track with a Z (see what I did there? eh? eh?), I wouldn't go adding mods to the Mustang. Not only are you sinking more money into it, but it usually won't help the resale value, imo.
Keep it clean, keep it pristine, make your payments, and keep it listed here and there in case you get lucky and can offload it for what's left on the term.
There's a reason people only suggest that you buy new if you intend to keep it for plenty of years: because you lose money otherwise.
Someone brought up the Mustang is better argument. Rut Roh! Now, if you were in a gt500 or a boss....
(In all seriousness, all of the above are great cars.)