Feeling happy....

Why do I feel so bad because I just can't seem to be happy in my marriage? It's basically no secret that I've never really truly been "happy". Am I just not trying hard enough? Is happiness something that we "feel" or is it something that we consciously have to "think" about. Why do I feel like I am letting so many people down because I just can't seem to "be" happy? Why is it so wrong for me to possibly want to end my marriage but it seems ok for everyone else around me? My brother was treated like shit for years by his psycho wife and after he got divorced, people celebrated. But just because my wife isn't a psycho, I'm suddenly a loser/jerk/deadbeat/dog because I just don't seem to be happy with her. She's done so many mental/emotional damage to me over the years that other people didn't witness so I'm looked at like the bad guy for wanting to feel free of her. How is that???