Hey, give it time. Starting with Dirt is the worst you could've done, there are warning labels in the game for a reason! Try to start with slow cars on plain tracks in Assetto, for example Magione with some street car. Try driving around for longer periods of time and progress through some cars and tracks with more and more elevation. I never had problems in VR, but some people get some really big troubles. It's mostly just a thing of getting accustomed to it, and some people take longer. Also try out reversing, but be careful, if you get naseous so quickly it might actually make you throw up (no joke, reverse in VR does that to people). But it'll train you. No reason to send it back - even loose_unit said that about his Oculus but kept it!
I returned it today, I got nausea that lasted even the day after. Longest session I managed to do was about 20minutes. The problem now is that I was spoiled by the immersivity of VR, and playing on a monitor/tv feels so... flat. I'd like to try a HTC Vive to see how it compares to Rift, the headset I got had pretty annoying god rays. I'm looking forward to the next gen of VR headsets, maybe higher resolutions and refresh rates will help in reducing motion sickness.
RSS just released their brand new GT1 mod (I LOVE that era of racing), which is a paymod with three (Viper GTS-R, Lister Storm, 550 GTS) of six cars released so far, but will get the other three (575, S7.R, Murcielago R-GT) at the beginning of 2018. Get it here, or just enjoy the sounds: