I hate hearing about the yo-yo or spot delivery as it sometimes called.
dude I hope your credit union can help you, but since you already know your credit is not the best, then you should know that 10 percent APR is the likely outcome.
I think you will find due to the terms of the contract (conditional or rescission clauses) it may be hard to simply "unwind" this deal and get your trade back.
On the upside you'll be aware of it in the future, and your not alone, this is the number one trigger of fraud lawsuits against dealers.
Yet they still pull it on those with less than perfect credit, hoping to jack the loan based on emotion. good luck