When I wrote my first novel I went through the gamut of emotions, but I knew why; the MC was based on someone close to me with whom I'd had a complex relationship, someone who had a big effect on my life. What I didn't know as I was writing it was that my subconscious had been telling me that I had to write it; first to be able to see these issues and then to try and understand them. Once I did - and it was a process marked by many nights of anger, sadness, revelations, and ultimate forgiveness - they lost their power over me. I forgave. I think it's a healthy journey. I think if you can find it in yourself to understand and then forgive, it will add years to your life. Just as when you write a story that's pure fiction (is there any such thing since everything comes from our experiences firsthand otherwise?) you have to have empathy for your characters. You find your empathy when you look at background, when you get to the 'why' people are what they are and do what they do.
On a loosely related note, just today I ordered a non-fiction book by an old friend of mine. A woman who lost her son and then her husband when the loss was too much for him and he took his own life. I know she had to write this book. She had a long period of grief and sadness and all the bad relatives, so it was a catharsis that she fought through and she's come out the other side smiling and with a perspective that will allow her to go on.
In short, I think it's painful but ultimately healthy, and as corny and trite as it sounds, forgiveness, it's the best thing for you.