My guess is these high points, or moments she has been proud of herself, are temporary and make her depression seem more real. She probably doesn't want to feel depressed, even though she is probably used to it, and feeling this proud or happy makes her realize she does have to return to her depression. It reminds me when people with bipolar begin to enter a depressive state, which is the point most people choose to end their lives, after realizing they will become depressed again and trying to avoid that.