|The best step is for you to add a new word to your vocabulary: "No." You went out of your way and assumed additional assignments to help your manager in the crunch, and all that happened is that you were crunched for doing so.|
The next time he approaches you with yet another assignment, you should not automatically accept it. Rather, let him know about the projects you are currently handling, and then ask which one or ones should be set aside so that you can take on the additional work.
It is possible that when he understands what you are doing and what must be deferred in order to assume the additional work, he might shop around for someone else to do it. And if he does indicate that one of your primary projects can be set aside so that you can complete additional work, that is something to keep in mind for your next review. Unless you take these types of actions, your workload will continue to go up, and your reviews will continue to go down.