It is not forbidden per se to have tattoos, and therefore you have not committed a sin. In our Shi'a jurisprudence having a tattoo is something reprehended (makruh).
You are not only blessed with following Islam, but also blessed with following a path that is founded on mercy and compassion. Almighty God forgives sins and wrongdoings, and in addition to forgiving He also wipes our slate clean. When you reverted to Islam, it was like you are a newborn, free of any sin, with no past. And you will continue to have such opportunities, as long as you stay enlightened in spiritual progress and work on your self to become a better person. Ultimately, this is the goal that all of us anticipate and want.
With prayers for your success.