The "German problem" is actually a documented phenomenon in research on language learning; it's called "language interference". There's also "language transfer", which is a more positive spin and largely on L1/L2, I think. Most research involves L1/L2 interference, but there are studies looking at L2 influence on L3. For most people learning a third language, their second language gets in the way much more than their native language. Funny how that works. 
I think that if you're coming up with German words less frequently, maybe Bulgarian is becoming your L2 and German your L3... which is good, or at least more practical, since you're living in Bulgaria!