Firstly, a few corrections that affect the meaning. In the first verse, "antiposition" should be "end position". The first chorus should be "And THAT I see a darkness". (It is the conclusion to a three-part question to his friend - more about that in a moment...) And the second chorus is "I'LL KNOW I see a darkness..." As to what it means, I see this song as a confession of depression to a friend. He starts by saying "You know me but do you know that I have depressive thoughts." He then says that though he loves life and people too much to actually kill himself, he does have suicidal thoughts. The three-part question is: "Can you see that a) sometimes I don't have that "drive to live" ("opposition"); that b) I think of suicide (the "dreadful end position") and that c) I suffer from depression (i.e. "THAT I see a darkness"). In the second verse, he simply hopes that, as their friendship continues with this new openness, it will grow and develop and he can find a deeper happiness ("pull the smiles inside") despite his depression. As the music reaches its brighter climax, he acknowledges his depression won't go away - but that by acknowledging it, both to himself ("I'LL KNOW I see a darkness.") and to his non-depressive friend ("my best unbeaten brother"), he can come to terms with it, and live with it. "This isn't all I see" means that the darkness, the depression is not all that defines him.