It's the same for me. I write much better English than I speak.
I guess the best way to learn speak good English is to speak English with other people a lot. Speaking with yourself is not the same. Don't be nervous. If you can't find the right words to say try to think of another way of saying it.
You write better in an unfamiliar language than you speak it because you have more time to think when writing.
I can understand written Spanish, and write it well enough to make myself understood... but that's because when reading, I can pause to puzzle out meanings, and while writing I can pause to think of a word without it being awkward.
When trying to speak it or understand someone speaking it though... I'm hopeless. Speech is extremely stilted and awkward, if I can manage it at all, and trying to understand the speech of someone else in Spanish is hopeless unless they go very slow and repeat themselves frequently and clearly.
When you're just a kid, you learn how to speak by ear and then learn how to write what you can speak, but it is different with foreign languages. When you learn a second tongue, you first learn how to write and then you learn how to speak what you can write, so there's your answer: you've gotta train, practice and then you'll have the same abilities in oral speech as in written speech. I'm learning French and I still have this problem. Despict of that, my oral English level is now reaching my written English level.
When you learn a language (any, first, second, third) , it's 4 areas that you need to develop, and it takes a different ability to master each area. 2 areas are of comprehension, 2 are of production.
oral(aural) comprehension: you understand what you hear.
Written comprehension: you understand what you read.
Oral production: you can speak in the language
Written production: you can write in the language
So you can be good in any of these areas, and a total failure in another, and it doesn't mean you can't at least be competent in one of them.
I find this very interesting, as I am a native English speaker who is learning Spanish, and I have the opposite problem to you- my spoken Spanish is good, my written Spanish is dreadful.
Also, Blaster, you may want to be a little more circumspect with your advice regarding your English ability, as you do not seem to have mastered the use of commas.
"I'm well trained in English... It is after all my Mother Tongue" should say "I'm well trained in English... It is, after all, my Mother Tongue." These details are also important for language learners.