I grew up learning Spanish in school (I'm from Arizona) and became fluent in Spanish. Later, in college, I started learning French. I noticed many of the linguistic similarities between the two languages and at first it helped me master the basics. However, I now find that as I am getting into the intricacies of French, it is influencing how I speak Spanish. It is interesting to note that when I learned Mandarin (I am at a high fluency level), I noticed it affected my Spanish when I started, but later my Spanish returned to its previous level. However, when the languages are related, I find the conflict lasts longer.
My question is this: What are some techniques to better differentiate between the two languages as I am learning more French to preserve my fluency in Spanish? Or will I have to go back to re-learn part of the finer details in Spanish?