I had made a brainwallet which i thought noone could have guessed and never put my private-key somewhere.
Sorry, there is nothing you can do.
The mistake you made was thinking that nobody would or could guess your password. No brain wallet is safe. There are 6 billion people in the world. What are the chances of one of them using the same password? What are the chances your password being guessed by any of the thousands of computers that are making millions of guesses every second? Both of those possibilities are very high. Any password that can be memorized can be guessed easily or accidentally.
OK, I realize that caution has to be used with brainwallets, but what you say is overstating the case. The 12 word mnemonics that have 128 bits of entropy generated by Electrum and others can be memorized, and so can be used as brainwallets. They can't be feasibly brute forced or guessed.