Exporting quality

Hey, so the guy who mix and mastered my song isn’t about to go back in to lower my adlibs. This is his second mix/master and he didn’t listen to the notes or call us while in the studio like we asked. He also claims he mixed it through analog or something and that there’s no individual files, just mp3s.

I decided I wanted to add what he fixed in the second mp3 to the first mp3 since the adlibs were lower then (better than being too high). I cut the song and dragged it into one mp3, everything sounds good in project. I export it in two different versions, highest and high quality, and both don’t sound as full or as good quality-wise as either mp3s he sent me. I figured since it was already mixed and mastered, I wouldn’t need to do anything? What would y’all suggest? How do I get it to sound the same quality (or at least just as good) as it is before it goes through GarageBand? It’s exported at its loudest already.