Amazing news ... Baltimore Orioles star Trey Mancini just finished 6 months of chemotherapy treatments -- and his GF couldn't have been happier, saying, "This is the best day ever!!!!"
Mancini has been battling stage-3 colon cancer ever since docs discovered a tumor last spring ... and the road for the MLB slugger to get better has been tough as hell.
Mancinci described getting chemo treatments amid the coronavirus pandemic as "crazy" ... and added he's been forced to receive his drugs through a mediport in his chest.
But Mancini's GF, Sara Perlman, said Monday the 28-year-old was finally done with his chemo treatments .