Hi, KarinaB, First, let me say that I am no expert on this matter, but what you've decribed seems a little over the top as far as their editing proceedures are concerned. Saying that, if this is a regular place for you to show your material, and the pay is good (as you say it is), it seems to me there is little you can do. You don't really want to lose this source of income/outlet for your material, so the last thing to do, in my humble opinion, is to have it out with them.
If I was in the same boat, I think that in future, I would only send them work which I was not too bothered about. If they "messed around" with it, it would not effect me too much.
Of cource, this is easier said than done when you have put in hours of work to get a piece of writing perfected; they are all your little darlings after all.
However, there may be a time in the not too distant future, when your work will be in great demand. Then you will be able to call the shots. Until then try to relax in the knowledge that every piece of work you send in, no matter how butchered it may end up, is another step to becoming a well known and respected writer.
chillies
