What helped me was quantifying my goals for the project. “Finish it” is too vague and daunting. Sometimes you’ll hit a roadblock and sometimes it’s just not going to work out like you hoped. Set yourself a deadline until which you’ll keep working on it before you decide you have better uses for your time. And within that deadline, set small daily goals like “work on project for X minimum amount of time” (once you start even for a short time, you’re more likely to keep going past that). If by the deadline you’ve lost interest, you can allow yourself to abandon the project without guilt because you know you’ve done the best you can. It only works if you are honest with yourself though.