Love needs reality. What is more terrible than the discovery that through a bodily appearance we have been loving an imaginary being? It is much more terrible than death, for death does not prevent the beloved from having lived. That is the punishment for having fed love on imagination. (Gravity and Grace, p.57)
These remarks sound deep, but they oversimplify the relation between love and reality. The reality love needs is not all or nothing --- in most cases, the person we love is partly real, partly imaginary. It is rare, if possible at all, for us to know and understand our beloved so well that we see her completely as she really is, without any distortions or fantasies. It is also rare for our beloved to be nothing but the product of our imagination, an imaginary being that we are attaching to the body of a person who we don't really know or understand.
Besides, it is not clear that imagination in love is all bad. A little imagination can be like light makeup: it makes our beloved look better without making her unreal. In love, sometimes fantasies can even breed reality.
Weil is still right that it is terrible to discover that we have been loving an imaginary (or mostly imaginary) being. What is terrifying, however, is not just the realization that the person we love has never existed, but also the realization that we have been so delusional. We may, because of the latter realization, lose our self-trust. If we need self-trust to trust another person (we need to trust ourselves in order to trust our trust in another person), and if we need to trust a person to love her, we may thereby lose our capacity for love. That's truly terrifying.