|Some managers will keep pouring projects on an employee until that employee comes up for air. The reasoning is that as long as the work gets done, the employee can handle a little more until something snaps. In this case, it sounds like you snapped at your boss.|
There are actually two reasons why you sense that he now views you as a marginal employee. One is that you flipped into combat mode as you adamantly refused to do another thing. Your approach may well have put him off. The second reason is that you may have changed your perception of yourself as a result of your actions in the meeting with your manager, and now you are interpreting many of his behaviors through this filter.
A more effective approach is to review your entire workload with your manager, indicate that some of it has to be delayed, and then ask him to help you establish a new set of priorities. This accomplishes the same objectives as the combat mode, but it does so in a constructive and businesslike way. Fortunately, you can still go back to your manager, express concern over the way you handled the first meeting, and then reset those priorities. And, doing this should also be a priority.