In order to use a car that you have shared (your own car) you must 'unshare' it. You can't even access the car at all until you take it offline which must be done in your garage (you can't do it from an online lobby). Not sure if that is something that has changed, but it's the way it is now. I kept making that mistake when I shared the R10 last week. I entered an online lobby to practice for the online race and had to exit all the way back to the home screen, then garage in order to get the car.