Why don't you create a payment plan with her? Go and get it notorized. This way, you pay her a certain amount and then you have actual money that you can budget out for things.. then it can be like a REAL car payment. If you want, you can pay it off early. If not, you can purchase the clowns with it. If she is iffy, just do a 2.5% interest rate. It's very low, but it sounds like she has no interest rate anyway.. so she may be more likely. It also helps you guys not be a one-sided relationship. Take my word for it.. it is WAY better to do this when loved ones loan money. Nobody feels ripped off either OR held back. My family is full of entreprenuers. I had a scholarship for undergrad, parents paid for grad school. The whole time, they paid living expenses. When I was done with grad school, I immediately wanted to open my own business. My mom loaned me $10,000 with my car as collateral at 2.5% interest and it all worked out great. We had it stipulated that I would start paying back and interest wouldn't incur until after 1 year (because businesses usually take about 3 years to get going and i needed to immediately invest back into the company). If anything ever happened, I called ahead of time and gave her a heads up that I was still waiting for a check from a client to clear, etc. BUT it was AMAZING because my parents didn't have anything to hold over my head... After the 1 year grace period I was allotted, I was able to pay it back in less than 2 years. BUT I had to be REALLY HONEST with them and the parameters I promised. You can get one drawn up at legalzoom.com for like ridiculously cheap if you aren't comfortable with the legal jargon.
But like I said, this is what my extended family uses with loved ones. Anytime it wasn't utilized, "disaster" occured. Generally someone becomes disgruntled with the other unless clear boundaries and expectations are set. Its not fair to her to watch you spend your money elsewhere when she is owed money and it is not fair to you to have this being hung over your head when hobbies are healthy. Just make sure you budget out your expenses ahead of time and know what you are getting into.
That is my 100% honest opinion. I've seen too many couples break up, clients feel jilted, etc. to not speak up. I just feel this is the most honest solution and will prevent problems in the future.