I made a promise to someone very dear to me which i may not be able to keep, and it will absolutely shatter them. I know it was my fault for making a promise I could not keep, but i don't know what to do, especially because I love this person with all I am to be honest and could not bear to see them hurt. I just feel so bad and don't know what to do to be honest. Any advice?
Don't make promises you can't keep...I know it's kinda a Captain Hindsight thing to say, but it just hurts everyone involved, including yourself when you end up breaking that promise, you'll just feel like shit. You need to go and tell that person that though they are still dear to you, it was wrong of you to make a promise you probably can't keep, and accept whatever the outcome is.
Why do you make promises what later you cannot keep? Rule number 1 in relationships: Don't make promises what you cannot keep in future. Well, there's nothing much to suggest, except talking to that person about how hard you feel and that you cannot keep that promise anymore. Try to think how that person will feel, when you'll brake that promise and consider if it's worthy enough to step such a big step.