Rob Kardashian is hellbent on making a comeback following his breakup with Blac Chyna and what's motivating him this time is another gal ... his daughter, Dream.

If you recall ... Rob started to get back in shape and healthy again when he began dating Chyna, but it all went to hell when their breakup drama ignited. Rob spiraled back into another deep depression and gained back the weight he lost ... and then some. 

We're told he's determined to lose it and has already hired a nutritionist and started working out again. His goal is to get back to the "old Rob" and he's determined to do it on his own ... with no help from friends or family.

As we previously reported, Rob now has 50/50 custody of his daughter with Chyna, and he's forking over $20k a month in child support, despite being broke. His family is footing the bill ... for now.

We're told his friends and family are skeptical he can rebound without their help, but they do believe he loves Dream so much he might have a fighting chance.