She probably got mad because she felt like she was helping/doing a favor, and then received criticism for giving her time to help you. I don’t think you’re in the wrong for wanting more control over the decoration process because you have a point, but it probably would have gone over better if you said something along the lines of “Thank you so much for your help! Would it be okay if I took the lead on the design since I’m going to be spending a lot of time here?”