All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
You did not mention to us what your mother forbade you from doing so that we can know whether or not it is permissible for you to disobey her in this. What we can say in brief is that your disobedience to your mother by doing what she forbade you from doing – if you have no right to disobey her in this – like if it is an obligation, for example – then it is considered undutifulness to your mother and this necessitates that you repent to Allah.
So you should seek the forgiveness of Allah, repent to Him by regretting what you did, and be determined not to disobey your mother again, and Allah, the Exalted, will forgive you.
Allah says (what means): {But indeed, I am the Perpetual Forgiver of whoever repents and believes and does righteousness and then continues in guidance.} [Quran 20:82]
Also, Allah said in a Qudsi hadeeth, “…and I forgive all sins, so seek My Forgiveness, and I will forgive you…” [Muslim]
Allah knows best.