Well it all depends, how good professional is, how much time you spend and how much you care about details. From my experience comparing to the jobs my nieghbord/friend had done by contractors - I can do better :) Though they didn't hire expensive contractors, I'm sure there professional who can do much better. But as mentioned in other comments, for average contractor time matters and they don't pay attention as much as you can. So if you care about details a lot and can't/don't want to hire expensive contractors, better do yourself.

Painting is especially easy, if you screw up some spot, you can always return and repaint (worst case you'll have to do that couple times :)) Painting equipment is (relatively) cheep, you can cover floors with plastic covers/blue tape and don't worry about screwing floors/doors while you learn :) https://www.youtube.com/watch?v=oNagIA8sKY0 watch these videos and you'd be ready to start. Cutting wall/ceiling line by hand is not that hard (the video has a trick), there're also "helper tools" for cutting lines in home depot/lowes. One trick is that if you go with light color on walls, perfect line with white ceiling doesn't even matter, you won't notice small deviations anyway.

If have doubts in your own skills (I did), try starting with closets and bedrooms. Only then move to living rooms/kitchens. If you screw up closet or your bedroom - it's not that of big deal. Though if watch above video series, it's really hard to screw up too much.