a lot more hands playing on-line, so the bad beats are going to happen a lot more often.. and they're memorable.. you don't remember the other 10 times your pocket AAs win a mid sized pot, only the one time they get busted
learn to get over it, or it will affect your play
plenty of people making a living out of on-line poker, but its a grind.. you can play +ve EV for a month and still end up losing, before the upswing