My suggestion to you is to show your fiance unconditional love no matter what his moods are like. Keep telling him that you love him, how much he means to you and offer your ear to any of his concerns or problems. By constantly showing love you'll help with his negative self-esteem during times of depression. You may want to do some reading about bipolar/ manic-depression to further your knowledge about this. As some one who suffers from bipolar disorder, I could try and help with any specific questions that you may have. But it's always best to be equipped with as much professional knowledge as possible.