you are correct that there are three voices to learn to discern: God, ourselves, or satan (and his demons). Actually i could add a couple more possibilities: Good Angels (our Guardian Angel for instance) and Saints (especially Virgin Mary). But the most important of course is God. Before i learned to hear the “still small Voice” of God i relied on the Words of Scripture, which come from God. Also you can rely on the teachings of the Catholic Church, which are infallible and come from God. For instance we know we are to love God with all our heart, mind, soul, and strength, and also to love our neighbor as ourselves. So at the same time we are trying to get good at this and the other clear teachings from the Bible and the Church, we can also take quiet time to try to learn to discern the “still small Voice” of God. Many of the prophets of the Old Testament could hear God’s Voice ("the Word of the Lord came to Ezekiel… etc etc) and also the Apostles and the Saints of our time (Peter: “You are the Christ…” Jesus: “Flesh and Blood has not revealed this to you, but my Father has…”).
To hear the Still Small Voice of God, you must be still. Jesus said to go to your prayer closet and shut your door. This is literal but also figuratively refers to shutting the doors of your eyes. Closing your eyes is very helpful to keep you focused. it may take a while and a lot of listening and patience, but if you want to hear the Voice of God you can. i believe it will pay off big for you. in the meantime, do what You know God likes. God Bless and i will pray for you :):):)