Lukas,
If you're having problems getting
temporary car insurance because of your age, then it might be worth extending the length of cover you require to make things simpler. What I'm saying is that if you get
1 month car insurance instead of just 3 weeks (which is an additional of 1 week insurance cover for yourself), then you wouldn't have any problems in getting insured. With
eCar Pay As You Go Insurance, you can get insurance cover for a month and can renew it afterwards if you need more time to sell your car.