How about starting both languages just a little, with short beginner-level books, then continuing with the one where you seem to be making more progress towards learning it and putting it to good use? In other words, try putting some of your perfectionist energy into thinking about efficiency and time management. That reminds me of Steve Pavlina, a self-improvement writer who's big on the Internet.
http://www.stevepavlina.com/blog/
Steve was a college dropout and kleptomaniac, then he started listening to motivational tapes about time management all the time and got a degree in computer science in three semesters and started a software company. Now he's known for promoting his experiments in intention manifestation, polyphasic sleep, raw veganism, and many other things that might help people become super-successful. However, from listening to an interview of him recently, I picked up that the only different things he started doing when he started being successful were: a) thinking of where he wanted to be in a few years b) listening to motivational tapes about time management c) choosing something to do that he wanted to prove he could do, rather than what others wanted for him and d) running 28 minutes a day. I think the running was most important for him to be able to handle a schedule of up to 13 hours of classes a day. Exercise lowers stress, so that when you have other things to do, you just do them and don't spend so much time stressed about decisions.
A person can only speak a language perfectly like a native speaker by living in the country and spending years studying. For instance, Hugh Laurie is a British comedian who plays an American doctor on television (House) who sounds like a native American English speaker. He can only do that by working in America and being coached on the American pronunciation of every new word his character has to say. So to learn both languages perfectly you'd have to spend a lot of time living in Greece and in Russia, talking with a lot of people there. That means your result will depend mainly on whether you find and pursue the opportunity to live and work closely with people in one or both of those countries.
