xn: I can sympathise with what you say. Someone who has studied Latin, and knows the etymologies of Romance words, may not have such a problem with getting words or pronounciations from one Romance Language confused with another when trying to speak. I wouldn't count on it though, and there is no written ancestral Germanic language, holding a place in that family corresponding to Latin's position with the Romance, that could be studied for this purpose, even if it would help.
After I finished Wheelock, I bought an Italian book for beginners. I had to stop using it because, since I had so recently finished studying Latin, the Italian (since it has a simpler grammar by comparison) kept looking like butchered Latin. It seemed that Italian was missing grammatical elements that should be there but aren't. Italian started looking like Latin that had been accosted on the streets of Rome, mugged, robbed of grammar, raped, stomped, hacked to pieces, and left laying in a pile of bloody pieces on the sidewalk. I could almost see the severed body parts.
I guess that a close attention to detail is necessary to keep from mispronouncing words from one related language to the other, but that's not easy to obtain or maintain when speaking face to face with natives, and having to spit out sentences on the spot. Learning to read a language is not the same as learning to speak it. I marvel at those who may be able to keep it all straight.