I am in a similar boat. My problem is that once I lose momentum on a project I have no interest in every revisiting the work I have done. Sometimes it is a full day of coding, sometimes it is a couple continuous days coding. Once I break away from the computer and start living life it feels like to much work to load all of the project back into my brain. I would finish more projects if I didn't have friends, family, a house, or a life and sometimes I wish for that.
I think that the problem is not that you lose momentum but rather that you start with momentum. I prefer having a clear mind and no need to rush (this happens when I’m excited about the project / have the initial momentum). This helps me a lot at deciding what I want/need/should/must do and what not. It also helps me to prioritize the project over/below family, friends, and house.
It helped me a lot that every time a have a new idea or want to start a new side-project that I wait at least two days before I dig deeper and get really started. Only then I really know if I really want this or if it’s just the initial hype/momentum.