The very first film I ever made was this fan film project. The whole thing snowballed from being a small community thing into this massive production with over 200 crew members from 30 countries. I had a love-hate relationship with my producer - they were brilliant but it was readily apparent that they didn't realise, when they took the project on, that I was an absolute rookie, and for this they hated me with the fire of a thousand suns. It was an abusive relationship that I had to deal with for the best part of four years.
Three years after filming the teaser with VFX for the four minutes still incomplete, about a year of which they had spent completely incommunicado, they tried to convince me to agree to cancel the project. The footage was owned three ways between me, the producer and the DP, who had long-since moved on from the project. I said no. They told me to think on it. I still said no. So instead, they said they wouldn't allow me to go out on my own and finish it. The film was canned. I was left hung out to dry.
Since then I've worked with some great people on a couple of documentaries and corporate bits, and I have come to realise that the pain I went through for all that is about as bad as it could ever get. I have recovered from that abusive experience and I am about to embark on my first "big" project since that one failed. But my *biggest* regret as a film maker, having been subjected to all that, the rigours of learning the film process, after investing time and money and all that jazz, was not having something to show for it.
Finish your film, whether you love it or hate it. It's a part of you, and if you do can it, one day you WILL regret it, for one reason or another.