Friend,
If you don’t go through the Catholic Church, as a Catholic you commit a mortal sin and the Church will not recognize your marriage as valid. It is possible to marry a non-Catholic in a Catholic church. You will need to see your priest.
Also, what about your children? Your faith is the greatest gift you have to give. Have you discussed bringing them up Catholic with your girlfriend? You need to think things over before leaping into marriage. You are in our prayers.
Fr. Vincent Serpa, O.P.