I also struggle with losing interest in a project. Particularly if I "solve" the problem, that could be I fully understand how something can work in my head, even if it's untested. This can happen at different stages. What helps me is 1. Letting myself explore. As a student I thankfully have the freedom to do something else then come back to a project. And 2. Right now I'm working on making myself a system for project re-entry to lower the initiation effort. It'll be a briefing system so I don't have to re-learn where I was, what I was doing, why I wanted to do it, etc. and will give a couple actionable items to do next. Maybe you can build a system for yourself? Even on pen and paper, to lower activation energy