Polyglot Problems

I speak Italian like I'm French, I spell Italian like I'm French. I think a native French speaker would speak and spell better in Italian than I do. 

I know, I learnt all about this, that despite the similarities between an L1 and an L3, the L2 acts as a filter to block the L1. It's still really annoying though.

My French spelling is so good, why do I have so much trouble in Italian when Italian is actually more similar to English and actually spelt the way it sounds?