i evade the cops and trafic fine but i get to the point and there is a cop waiting for me so then he changes place and i get cops on me drive faster and turn alot and then the cars wrecked like that so should the cop be there and if he isnt how can i get rid of him? and if he is how do i do the next part of the mission?

just drive as fast as you can, ignore the cops as much as possible dodge when they try to ram you but don't ram them back in the hope of taking them out- it just damages you more, get to the first location avouid getting the cops to chase you here as it is possible not to. when the location changes it is inevitable that you will get chased so just keep on driving, brake as little as possible to prevent the cops overtaking you. when you get to the drawbridge just keep on driving if the cops are far enough behind you will pass, it may take a few atempts.

hope this helps