I understand completely.
My DH wanted to get busy and start remodieling my sewing area. He even drew up plans for the wall unit. It doesn't even come close to what I need or want except it takes up the whole wall from side to side and floor to ceilling. Told him I was still undecided as to what I reallly needed, appreciate his plan, and maybe we should sit down together to figure it out, and that I was in no hurry to get the project done. He was ok with that.
Let him know in a gentle loving way but firmly that this is your room and he needs to listen to what YOU need in there. If he refuses, simply tell him you have changed your mind and use it once again as a bedroom. If he asks why, tell him you would not enjoy working in your dream room where none of your dreams came true. He will want to please you.