Praise be to Allah.
It is haraam for you to wear this bracelet, because bracelets are jewellery for women, whether they are made of gold, silver or anything else. So it is not permissible for men to wear them. Rather it is permissible for them to wear rings of silver.
If a person makes a promise to someone to do something haraam or not do something obligatory, it is not permissible for him to fulfil his promise.
It is not permissible for you to fulfil your promise to her to wear the bracelet and never take it off, because it is a promise to do something haraam.
If a vow to commit sin should not be fulfilled, and in fact is not permissible according to the consensus of the Muslims, as Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (10/69), then it is even more appropriate to say that a promise to do something haraam should not be fulfilled either.
Please see the answer to question no. 30861
Gifts are of two types:
Those that are purely gifts and are given as a token of respect or friendship. These become the property of the recipient and it is not permissible for the giver to take them back.
Gifts given with the intention of getting something in return from the recipient, which the fuqaha’ call “gifts for which there is the hope of recompense or reward”. In this case, the giver may take his gift back if he does not get what he wanted.
Based on that, if this girl gave you the bracelet so that you would carry on your relationship with her, and you ended the relationship, it is permissible for her to take back her gift. If she does not come to you concerning it and does not ask for it, then there is no blame on you and you do not have to give it back to her; you could give it to a woman to wear if it is suitable for women, or you could sell it to a jeweller or give it to him to make rings for men or jewellery that is suitable for women.
And Allah knows best.