Please try not to minimize your own feelings by saying that others have it worse. Bunny trail this line of thinking and nobody has a right to be sad/upset/depressed except for the one person on this planet who actually is worse off than everyone else. You have a right to feel this way, and reasons for feeling this way. It's not just a matter of making the feelings go away. It sounds like you need help (in terms of actual support in taking care of your niece and perhaps therapy too).