I used to have the same problem, you generally break your promise to do something because it's either boring or not very rewarding. So, if you continue doing the thing, in the end it gets you stressed and you want to go back to your comforts again. Here are some things that helped me stay in focus.
You need to find the right reason, like, why am I studying? of course the obvious answer is "duh! exams?!", but it needs to be something that's more meaningful to you; like for me it was because I was not good at anything, so I wanted to be the best I could in studying. You'll have your own reason too- Find it
Allocate a specific time to study every day. If you're planning 6 to 8 pm, then just cancel every activity you have on your schedule and hit the books. If two hours don't suffice then go for 4 or five hours, generally that's the amount of time you need for college level (more than sufficient if you want a good score), but just don't overdo it. Doing this everyday will bring a sense of routine in you and it will make you focus and with practice- you won't think of anything else other than studying during that time, so it's imperative that you study everyday at that particular time (just keep the book open at least :P).
This step is kinda hard (It's hard because you're changing something in your lifestyle, and your body resists change- ever heard of inertia?, but once you get it rolling, it gets easy) when it gets too hard, just remember the reason (step 1?) why you're doing it and keep practicing. Practice makes perfect so- Practice it
You're a human being, so it's natural to waste time on things you like- healthy even. Friends can be a pain sometimes, so just draw a line, let him/her know their limits and tell them off if they invade your private time (like if they call you during your study time or if you feel like calling them, then just say a plain "NO" in either case. Better just to switch off your phone. You can catch up with them after your study time). Things like movies, games etc seem like traitors only when they prevent you from doing your work. So if you have a chapter to finish in four days for example, allocate what you will study on each day and leave a few hours for entertainment on each day, so, you know that everything's going according to plan and if all goes well you'd have completed the chapter by the fourth day. This way your work gets done and you get to have a good time too!
So basically, step 3 eases the difficulty of step 2!!
In short, it's all about balance, time management and introspection-
balance- balance between things you do purely for fun (TV, sports, hanging out etc) and those that you do for a living (that's usually boring because it involves studying if you're a student or a job if you're working)
time management- manage your time so you can have that balance on each day
introspection- to know yourself, what are you inside. Meditation usually helps for this. The awesome thing about introspection is, it helps you control your emotions and not the other way round.
So, live life, be philosophical, if this were your last day in the world, would it help if you think about yesterday and all you did was studying!! :D
Hope this helped, good luck!