Trike vs Rex

Yes its another rex rant. Yes I'm a bad trike player and I'm still learning on the PVP server. But why man even if I get the rex low to 20% - 30% HP and even as high as 70% or 80% as long as they fracture me all they got to do is wait every 5 seconds and whittle me down and I can't do anything about it? I try my best to time my attacks but they can just cancel it? That seems unbalanced. Its like having 4 bars of HP every fight.