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I think you should practise talking more to people and ask them to correct mistakes everytime you are wrong. I see some of my friends succeed by this way. Besides, even natives sometimes speak incorrectly. Don't worry too much.
I'm originally from Romania, and I've never studied English in school, but I started learning it on my own when I turned 18. I'm 25 now, and I still make mistakes, although not as many, of course. :) How did I manage to learn English and improve it over time? It's actually quite a simple approach that I used, but it could be difficult to follow, because you have to keep up with your exercise, trying to speak as proper as possible. My wife is American, and she always corrected and explained what I'm doing wrong, etc. I tried to understand and use that understanding in my next conversations (written or spoken). I certainly don't think that English is a hard language to learn, but I can say that like any other natural language out there it has exception about it, and that's the pretty significant difference between pronunciation and writing. To me, that's what makes English difficult. Other than that it's not hard at all, although like any other natural language out there could become really hard depending on the level and subject you're trying to talk/write about. ;) But this is not a great concern since you're just looking to speak more proper. I would say to start understanding verbs (their forms, etc.). It's all about how you're trying to form yourself and how careful you're with your language. Language is the most important factor of our living (it defines you as a person). Good luck, and in order to get your practice try to start writing things either for yourself or for others (like a blog about something that interests you, etc.)
Writing will help you speak better too. Then try to speak like you write.
The biggest advice I can give you? Read. Reading will get you familiar with the patterns of the language, and it will form a concept in your mind of what `sounds' right and what `sounds' wrong. That's what your brain will fall back on when you're trying to assemble sentences on the fly, as when you're speaking to someone. So yeah, read read read.
You need to read more books and refer the dictionary if you are not sure about he word. You may also visit some online grammar sites via internet.
Absolutely! Reading several programming books is what got me to where I am now as far as my English (reading/writing/speaking) goes.