Paradox: "Cause I saw the end before we'd begun" We cannot literally see the end of something before it happens, we often have feelings on how we think something could go. He is stating the he saw conflict and thinks of the worse situation. And even though he has bad feelings about how this relationship will turn out - he preasues the relationship, because minus the conflict that leads the couple to the end he would rather experience it all with her, because of his strong feelings.
Personification: "And love is blind and that I knew when, My heart was blinded by you"- Love cannot be blind. Often the time when people use the phrase "Love is blind" it is applied to a couple that is avoiding truth of some sort or it also can be applied to a lover that is blinded by their lovers faults & imperfections. Knowing this James claims that because love is blind he knew because of her his heart couldn't see the faults & imperfections that lead to their break up.