If you had made a promise then you should try to keep it by any means.If its a vow to God ,done ritualistically, then not doing as promised is even a greater sin as per our scriptures.
Please refer to description of sins and the related Hells (Narakas) found on this page in Sri Devi Bhagavatam.
On that page you will find descriptions of what happens if one makes false promise under various circumstances.
Quoting from the above link:
Now if anybody makes a false promise or swears falsely, taking the
Tûlasî leaf in his hands, if anybody makes a false promise, taking the
Ganges water, S’âlagrâma stone, or any other images of God in his
hand; if anybody swears falsely, placing his right palm on the palm of
another; if anybody swears falsely, being in a temple or touching a
Brâhmana or a cow; if anybody acts against his friends or others, if
he be treacherous or if he gives a false evidence; then all these
persons go to Jvâlâ Mukha hell, and remain there for fourteen Indra’s
life periods, chastised and beaten by My messengers and feeling pain
as if one’s body is being burnt by red hot coal. One who gives a false
evidence, with the Tûlasî (holy basil) in his hand becomes a Chândâla
for seven births; one who makes a false promise with the Ganges water
in his hand, becomes a Mlechcha for five births; one who swears
falsely while touching the S’âlagrâma stone, becomes a worm of the
faeces for seven births; one who swears falsely, touching the image of
the God, becomes a worm in a Brâhmin’s house for seven births; one who
gives a false evidence touching with the right hand, becomes a serpent
for seven births.
Also refer to this page and please read what is written regarding taking a sankalpa(vow) before chanting any Mantra.
Quoting from that page:
If you have taken a vow to do X number of japas in Y number of days –
jolly well do as vowed! No short cuts and no excuses. This is one
failure for which the Gods make you pay! So think and calculate
carefully (use a calculator please!) and keep an emergency cushion,
before you take a vow!!
So,in short,making a vow and not fulfilling it is NOT something good and more so if its done in a ritualistic manner(like taking vows touching Holy objects,or while taking Holy water in hand or making vows with the appropriate hand mudra(the sankalpa mudra)).
But having said that,don't worry.In our Sasthras there is a remedy for every problem.There are prayaschittas for every sin.
Prayaschitta is done for the destruction of sin. In the Code of Manu
you will find various kinds of Prayaschitta for the destruction of
various kinds of sins, such as the murder of a Brahmin, killing of a
cow, drinking alcohol or taking forbidden foods, adultery, etc.
Prayaschitta is of two kinds, viz., 1. Extraordinary (Asaadharana) and
2. Ordinary (Saadharana)..... Complete fasting on Ekadasi and Pradosha days destroys many sins. Every one of you should practise
this. Bathing in the Ganga, Japa and ordinary pilgrimage constitute
ordinary Prayaschitta for the destruction of small sins.
The above prayaschitta measures are taken from this page.