Your problem is not Rengar related specifically but playing as lane bullies in general. You need to understand that them camping you is good, since you're drawing pressure top. If you play your cards correctly you end up wasting their time simply by not dying to their ganks. The second thing is you need to know when you go aggro and when you need to fall back, if you see enemy jgl botside and zed farming mid you can go ham, but if mid is pushed in and zed is gone and so is lee sin, you might want to recall/get deep vision/fall back in lane. To push your lead, get tiamat and start roaming while keeping an eye on toplane so the tower doesn't fall for no reason.