A problem for me has been the inconsistent nature of intrinsic motivation. I start out in that "honeymoon" phase where it's fun and interesting, but after a few weeks or days (or even hours in some cases) it loses its intrigue and becomes boring work. So I either wind up forcing myself to continue or adding it to my pile of unfinished "to be continued" projects. (And knowing that this loss of intrinsic motivation may happen feeds the fear to even start new things.)

(This is to say nothing of the inherent problems with extrinsically motivated work, such as freelance work.)