Your distress shows the difficulty of expressing transcendent concepts in mere words. The truth is, we don’t know exactly what heaven is like—even if we did, we could not reduce it to language, which is a human device incapable of containing God.
Your group leader’s statement that heaven is “in our minds” is misleading, however. Heaven is within us, in the sense that each Christian is a temple of the indwelling Trinity. But it is no more accurate to say that heaven is only within us than it is to say that, by virtue of our receiving the sacraments, Jesus exists only within us. God and heaven live within us—but they also have an objective life beyond us as well.
God, in a sense, is heaven. In 1 Maccabees 3:18, for example, the author uses “Heaven” as a name for God (to avoid using the name it was forbidden to pronounce). Heaven exists wherever God is. Jesus said, “The kingdom of heaven is at hand” (Mt 4:17), meaning that, since God took on flesh, heaven itself is here, among us. After death, our experience of heaven will be to enter fully into the intimate love of the Holy Trinity, to our everlasting and perfect joy. To the extent that we participate in that love while on earth, we begin to share the joy of heaven now. Catherine of Siena said, “All the way to heaven is heaven.”
You are right that the Bible refers to “heavenly places.” We must keep in mind that time and space are finite concepts; God is not bound by them, nor could God live in one physical place, as we think of it, because the physical universe is his creation and cannot contain him. Heaven is a place, but not a particular space.
We must understand the symbolic language of the Bible and the saints as an attempt to communicate the ineffable. We do not understand literally terms such as seeing God “face to face.” We rejoice in their meaning: that we will be with God, intimate with him, unhindered by our mortal limitations.
As the Catechism says (section 1024), “This perfect life with the Most Holy Trinity—this communion of life and love with the Trinity, with the Virgin Mary, the angels and all the blessed—is called ‘heaven.’ Heaven is the ultimate end and fulfillment of the deepest human longings, the state of supreme, definitive happiness.”
We leave up to God “where” and “how” he fulfills his promise.