Irena.
I sympathise with your predicament, but without a firm diagnosis from a code reader we can only guess which of the many possible faults are responsible for the problem. Those possibilities include a blocked diesel particulate filter, a dying battery, faulty fuel pumps, faulty mass air flow meter, and several others as well. It is clearly uneconomic to renew every possible part in the hope that you will be lucky.
Audi should be able to diagnose a car as young as yours, and I am puzzled your workshop can not do so. Is there an alternative Audi workshop you could try? Are there any independant VAG specialists you could try? (Many of us here prefer independant workshops for reasons I need not mention here, but I do not know if they exist in Germany.)