I'm not an expert by any means. But I have recently been in a very similar situation. I feel your pain....It has been REALLY frustrating!! I have been boarding for 10 years (with a 3 yr break in the middle) and jut got a new board that I CAN NOT ride! Well, unless its steep and the snow conditions are perfect Having said that, I think I'm coming out the other side and I think there has been some really great advice given here. Here are a few I totally agree with!
1) Technique- If the size, length and width are right for you, it really is probably just a matter of technique. You learned to move your body to manipulate a different board. With this board that same movement isn't working any more. I would think the first thing would be to find out if there I something wrong with your technique and if its gong to be a learning curve thing or if the board just isn't a good board for you.
2) Mental barrier-Someone jokingly mentioned a mental barrier. But seriously, with my new board I found that I didnt like feeling out of control so much that I was a really timid rider on it. I didnt want to get hurt! So i went went way slow and spent all my time on much fatter terrain. I did his even though I know its MUCH harder to snowboard, initiate turns, control your board etc, when creeping along AND on flatter terrain. I actually wrecked HARD as a result. I totally had a little mental something going on because snowboarding is one of the only things I truly love in life.....and I wasnt loving it! All I knew is I did not like the way I felt on that board and I dint know why!
3) Which leads me to my last point. Do a TON of research and then go demo a bunch of different boards so you can identify what you like and don't like and why. That way youre not at the mercy of whatever board you blindly end up on. I'm grateful for this mess because I have learned so much about my riding style, what things I dig in a board, how snow conditions and my mental place effect those things, and different snowboards in general function. Turns out I think I had to much of a board for me. It was too wide, too stiff, and just too different than what I was used to.
In the end it's supposed to be fun. You need to be able to find a board you enjoy. Don't get too caught up on the th things people say you should like etc,. And if you decide this board jut isn't for you, you can always sell it.
Good luck....an sorry about all the typos.