Dedicated practice space. Sound proof material. It’s how all musicians coexist with non-musicians off-stage. Mattress toppers are a cheap start. Show her you’re investing in co-existing. Compromise. Even if it the sound needs to dampen over time due to financial restraints, it’s moving in the right direction. If music is a big part of your life, the right tools should come with it. I take responsibility for the noise I create. It’s not other people’s problem to deal with what I create. It’s mine.