No wonder you're losing, with that kind of attitude, you can't win open world PVP or arena PVP no matter what kind of class you play. I understand that it might be frustrating, but what good will it do complaining?
They aren't going to nerf assassins just because they're stronger in open world PVP. The arena is the main area for PVP. Either play arena or stop complaining. There's nothing they can do to fix sins in open world PVP.