It is far easier to break a promise to yourself than a promise to others, because you know you’ll be able to handle the disappointment when you find out. Even if you are destroying self-trust, isn’t that something you were supposed to lose a long time ago?
But the reasoning goes the other way: whether you break a promise to yourself or you sin against God, it is pure selfishness. You are centered in the moment, in your wants, your ego. You should be stuck on the future, on the unconditional love God has for you, and the intense love you have for Him. Sinning against God does nothing more than hurt yourself and drive yourself further from Him. Your sin doesn’t hurt Him as much as it makes you mistrust yourself and mistrust His love for you in spite of your actions.
I broke promises to myself and to God this week. I sinned deeply. But I turned back to Him. It will take a long time until I can trust myself in that area again, and the love I have for God has been wounded, but Christ has forgiven me, and He loves me. I want Him more than anything, and my eyes are set on that goal.