First of all, and for the case it's a promise to abstain of bad thing (as breaking of a promise to abstain from bad coming with it, wouldn't be kammical a bad thing): Sadhu for openness and considering a fault, wishing to do things right, of which is actually very praiseworthy, making you a serious conducting person for many good people and a good example of acting for many others, that things can be solved by confessing and strong determination not to do it again.
The teachings to Rahula, the Sublime Buddhas will be good encouragement, starting about lying and guide for all days actions.
Yes. Twofold, no threefold, no fourfold grave, good householder's daughter. One is the cheating, breaking promise toward your mother, a child's first God. Second, it was a promise to do what is good, a 'holy' promise, for your own support, breaking which turns foremost against yourself, maybe not seeing the disadvantages of what the householder's daughter went for. Third, hurting oneself, causing long bad periods of consciousness, in violating a fundamental precept. And fourth, at least, 'cheating' the Buddha, of which can fast turn one away from the Jewels, requiring renewing refuge to walk on a good way.
So, but all of that can be solved by best confessing as fast as possible (both, your mother, not to speak of the Buddha, will highly praise your deed if doing so, and accept it), renewing the promise, best taking five precepts seriously, and keep a strong determination to follow it steady in mind. Confession itself has actually already been made by the question, and surely that gave already good release -- but also good, for good relation, to ask for pardon from your mother, and best doing it as well toward the Buddha, and the Jewels, a little cleaning ceremony, which brings sure a lot of joy and good self-estimation.
As for reduce the danger to do it again, best cut of ways of access to it -- including bad friends -- and inform yourself about the disadvantages of it: best if approaching wise people in detail of it.
It's totally the way the Buddha's path to liberation, peace, works: by confessing and steady better new begin, cleaning is it called, or lifting oneself out of a hole one has fallen into, by doing so.
Best wishes and never be shy in doing what is good. Mudita
[note that this has not been given for stakes, exchange, trades, to be used to bind, but for escape from this wheel]