dagypsy - I understand when you say you feel demoted. We have been together for 19 years and that is exactly how I feel. I sometimes feel like the boring wife he has to come home to and she is the fun, exciting, sexy one. The way my husband tried explaining it to me at the beginning was, sure, it's fun to go on vacation and stay at a fancy hotel for a few nights because it different and new. But after a few days, you are ready to go back to the comfort and security of your home, your bed. I am his home. This is where he will always come back to. It's taken me a while to believe it - but I finally do.
I agree with ak2381 - it does seem like a little soon to be going away for the weekend. Has he had any overnights with her first or is he jumping right into a weekend? We have been in this situation for a year and so far my husband and his girlfriend have had 2 overnights and a few daytime dates. They are talking about a weekend away - but they aren't rushing it. Overnights are hard enough - but they are good baby steps. The first overnight he had with her, I cried when he left, I cried when he was gone, I cried during the night while I slept hugging his pillow, I cried the next morning when I woke up to an empty bed for the first time, I cried when he came home and I cried off and on for the next day or so. This last time they had an overnight - we had fantastic, kinky sex the night before, I hugged and kissed him before he left, I kept myself busy while he was gone and I gave him a big hug when he came home. There was ZERO crying this time around! I'm not sure if I am quite ready for a weekend yet - but I know it's inevitable.
My best advice, if he isn't willing to postpone this weekend trip until you are a little more emotionally stable and able to understand the situation, is to keep yourself BUSY, BUSY, BUSY! You can't sit home and cry the whole time like I did - it sucks and does nothing to change the situation.
As for cheating etc, I don't see this as cheating, as my husband has been upfront and honest from day one. It would be much worse if he was sneaking around behind my back and lying to me about where he is going. I always know when they are together, though I don't want any specific information. The key is honesty, openess, trust and communication.
Hang in there - freaking out is normal, but will ease with time. You are not alone!