It depends on the nature of the promise.
I promise to be faithful to my spouse, that is a promise made to both God and my spouse. To break that promise is a mortal sin.
If I promise God that I’ll pray a rosary every day, and then I forget to, that is not necessarily a mortal sin. It’s not even necessarily a sin, sin requires intent. Simply forgetting is not intent.
Could you tell us the nature of your promise? That may help us help you.
That being said, as a high-anxiety person you need to be careful not to fall into scrupulosity. God isn’t sitting there waiting for us to sin so he can yell “gotcha.” He’s a loving, infinitely patient father who desires our success.