I'm sorry you're feeling so bad right now.
The only thing I can think of is maybe getting an instrumental version of a song that people will recognize as being an erotic song, a song that you are already familiar with, so that it will be easier to write to? Also, if it's music that the audience is familiar with, they may really get into it because that song will bring up great memories.

I don't know what your style is like or anything like that, but if it's kind of jazzy, you could do a search for jazz musicians who have done a song like "Between the Sheets" for instance, and then call around to local stores to see who carries it.
As for your mom, maybe since she knew that you would be in front of an audience, she wanted her voice on there so that she could be heard also? I don't know, but I know it's upsetting for you. It would be hard for me to write anything erotic to something that had my mom's voice on it...talk about a mood killer! For now, focus on getting everything together for your performance tomorrow, and then after all of that's over have a talk with your mom and let her know how this has made you feel and what kind of pressure it put you under.
I hope that everything works out well!