if you are playing the midi, presumably you have the expression and phrasing you want so its a good perfromance you just don't like the sound.

what synth are you using? maybe a better synth would sound more like the real insrument you want.

some midi synths don't sound realistic. if you use sforzando you can load some very convncing sounds into midi.

what instrument do you want the midi to play and what synth are you using at the moment?