It depends on how much you knew about the faith when you left the Catholic Church and whether all of the conditions for mortal sin were met. If all of the conditions for mortal sin were met in you leaving, and if you die without repenting then your soul would end up in Hell. Did you make a diligent attempt to learn and understand all that you can about the Catholic faith before coming to the decision you made? Did you leave the Church because of pressure from someone else? Was your conscience bothering you but you left anyway? Since the time that you left do you miss being able to receive the Eucharist in Holy Communion?
If you regret leaving, you can return by making an appointment with your local parish priest. And information and assistance is available at a website called
Catholics Come Home. Was your reason for leaving that you are of Jewish ancestry? If so, I recommend a website called
Association of Hebrew Catholics.
"Mortal sin requires
full knowledge and
complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin." - Catechism of the Catholic Church, 1859
The 4 Last Things: Hell - by Fr. Wade Menezes, CPM - YouTube
References:
Catholics Come Home
Association of Hebrew Catholics
Mortal Sin and Venial Sin
Pope St. John Paul II teaches about Hell