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, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
We have previously underlined in fatwa 271513 that it is prohibited to give a Muslim the name Rahmaan or any other name that is exclusively indicative of the unique attributes of divinity (that only befit Allah) or to keep it as a family name, and that it is an obligation to change it.
The one who gave you this name bears the sin if he was aware of the prohibition in this regard. As you have grown up and have come to know of the prohibition, you bear a sin for not changing it if you are able to change it and have not done so.
Allah knows best.