St. Thomas writes the following about attaining true happiness:
[M]an cannot attain his end of Perfect Happiness by his own powers, but only by God’s grace. [ST I-II q.5 a.5]
The Throne of Grace
Why? Because for man, perfect happiness comes only through seeing God Himself. Aquinas refers to what St. Paul writes in 1 Cor. 2:9: “Eye hath not seen, nor ear heard, neither hath it entered into the heart of man, what things God hath prepared for them that love Him.” The vision of God is beyond our natural powers, both of sense and of mind. He addresses the point that seeing God’s Essence is our true happiness in ST I-II q.3 a.8, quoting St. John 1:32: “When He shall appear, we shall be like to Him; and because we shall see Him as He is.
Aquinas makes this same point repeatedly in the Summa Theologiae and Summa contra Gentiles: There is no possible way that our natural powers are sufficient for us to get to heaven—to see God—on our own. There is no question of deeds we might do nor of any natural merit that we can ever do or ever attain that can achieve this. To the contrary, Aquinas insists (and so has the Catholic Church throughout the ages) that we can only ever see God – we can only attain to heaven – by the grace of God. There is nothing in us whatsoever that can change this fact, even if we had never sinned. But we have sinned, and that additional problem makes things all the more impossible.
Aquinas is not presenting some fancy idea of his own. He is merely explaining what the Church has always taught. Those who say that the Catholic Church teaches we can reach heaven on our own are simply mistaken.