Solved

Car parts

I believe it may be linked to your Design HQ. The higher the level of the design facility the more parts are used per race. What are the levels of yours and your girlfriend's?

Yes, the reason your cars take more parts to repair than your girlfriends is because your Design HQ has been upgraded to a higher level. This means you also need to upgrade your manufacturing facility to keep pace. 
From my own experience, I have never had problems maintaining a good stock of spare parts. My daily ritual includes watching 5 ads and currently my L15, two car team that I created in May has a stock of 600 parts. It helps that the league in which I compete doesn't race on Friday or Sunday, so that's two days a week when I receive parts for Ads but don't consume any.

You can also do the "Abu Dhabi trick" which consists in not repairing your car for this race (you can include Brazilian iGP as well if you want).

This trick does work while car condition isn't take into account to simulates race pace, only engine condition is.

Actually the game is pretty well balanced, it usually takes up to 20~24 parts to fix two cars.