My song doesn't sound the same when I bounce it, no matter what I try. Why?

I created a song on Logic Express and I like how it sounds. However, whenever I try to bounce it, the bass sounds super fuzzy, some things that were quieter are much louder, and some things that were quiet are quite loud. I just want to maintain exactly how it sounds when I play it through logic and convert it to MP3. I have trying for a long time and I'm quite frustrated 😟 Any suggestions would be greatly appreciated!!