This happens with me a lot. I see that I need a catalyst because I can't usually do things by myself. Want to lose weight? get a workout buddy? Need to do well on a test? Study with a friend! Find someone you trust to give you the push you need in the right direction!
Try rewarding yourself! I usually think of an end goal that would make me really happy or say if i do this then i will buy myself some Starbucks or some yummy food. I also think tater is right, maybe do these things with some friends and make it more interesting. Good luck!
I find that oftentimes the reason I can't finish things is because I get discouraged when my expectations don't match up with my reality. I think that it is important to be realistic about how you set goals for yourself. For example, if you don't lose weight as quickly as you want to be, instead of wasting energy obsessing about why you aren't successful, try to think of alternate pathways. It's also possible that you might have ADHD and you naturally have poorer impulse control than the average person. In that case, your best option maybe to talk to a psychiatrist. Even if that isn't the case, talking to a qualified professional can really help.