I get that. It is incredibly difficult when you look at it like that. I have personally felt that pain of, "So, who I do I take time from, my wife or my kids?"
However, in the case you mention, it is only 4 or 5 months of nights and weekends. What is that, 30 actual days max? Would you trade 30 days of the thousands and thousands in your life to be able to make something you are proud of and potentially allow you to spend all of your time doing what you love? I know I would.
Obviously, thinking like that can lead to a workaholic, Cats in the Cradle (google it, young'ns) type existence, but with measured use of it and diligence at maintaining boundaries, I think it is possible.
I remember when I had a friend ask to me run in a 5K. Before that, I wouldn't run unless being chased, and even then... However, I realized as much as a pain as it might be, even being slow 5K is about 30 minutes. No matter how painful, annoying, embarrassing, it is, that's half an hour. 2% of 1 day. I decided right then that I could do anything for 30 minutes - and did. This is the same sort of thing.
You can do anything for 4 or 5 months and see what happens. This IS your life, and it's ending one minute at a time.
</pollyanna boosterism>