If neither the commentators nor I (meanwhile) are happy with my question, should I edit it completely or post a new one? (how do I close the fist one then?)
should I edit it completely or post a new one
You should decide whether it's about content or about presentation.
If you just want to rephrase the question to make it clearer — then, please, don't post a new question; edit the old one (even if it requires a complete rewrite, more or less).
But, if you decided that you actually want to ask a mathematically completely different question — then it should be posted as a new question, obviously.
And if you really want to close some of your questions just flag it for moderator attention.