How to learn FRENCH and not forget my SPANISH?
I have worked really hard at being almost fluent in Spanish and recently I've taken a large interest in French. Lately I've been focusing more on French because its new and interesting, but I don't want to waste my Spanish skills. I tried learning French through Spanish which was working, but I have been finding that the best sites with the best activities are ones where you can only have them in English while you learn French.
What should I do to learn French, but keep my level of Spanish?
Assuming your native language is English, continue to study French on the English websites. The perspective will be taught from the perspective of native English speakers, and therefore be more meaningful to you.
This is important because you say you are "almost" fluent in Spanish. I don't know what your definition of fluent is, but if you mean you can almost carry on a conversation in Spanish, that is not the same as being academically proficient in Spanish. Academic proficiency is being about to think "about" the language, and not just think "in" the language. It means you understand the relationships between parts of speech, and you can do literary analysis in the language and have serious discussions (not simply everyday social conversation). You should use YOUR strongest language to help you learn the new one, especially since Spanish and French are too similar and you could confuse grammatical differences.
I wouldn't worry to much about studying the two simultaneously, because you have gained a level of Spanish that will keep you from confusing the two languages. I never recommend studying two languages where you don't know either of them at all. I like to see a student take two years before starting a new one, and if you think you are almost fluent, you are probably beyond that level already.
French structures are VERY similar to those in Spanish, but there are some differences. So as you take off in French, you will easily understand things that someone who has never taken Spanish cannot understand. Now French pronunciation is COMPLETELY different from Spanish, and it's vowels may have more than one sound like in English. So you will find some easy concepts, and some challenging ones.
It sounds like you are just the type of person to be successful in both languages. I congratulate you on your perseverence, which I feel is one of the most important traits to being able to learn a language well. Your time will pay off. Have fun!