[alvar0]

You need to figure out why you and your project only get to 25-50% completion. Address the issues on your side and pick projects you're more likely to finish. The biggest motivator is doing something you like. The biggest "demotivation" nowadays is being young and used to constant rewards for doing tiny tasks (or nothing at all)- of course you'll find hard work with no reward over a long time horizon impossible to do.