Often, when I'm struggling to find energy for after hours projects, I find that it's because I'm neglecting ancillary tasks such as eating nutritious meals, exercising and giving my brain down time (usually these can be combined by cycling etc), and planning my work in advance so that the barrier to start each day is low. Other times I'm overwhelmed by stress or sheer number of tasks (which can again be mitigated with more planning)

A sibling comment says that lack of energy indicates low motivation and that you should find a new project. With due respect to the commenter, this is not useful advice. If you are interested in completing a project, you have enough motivation to do so - what you may not have is the right environment around you. So identify what's stopping you from working, and get rid of those road blocks. Some times they aren't obvious and can take some time to identify, but keep paying attention and eventually you will spot them.