There are various stages or phases that a project goes through to bring it to completion. These typically involve some form of Initiation, Planning, Execution, Control and Closure.
Why are you managing lots of projects at the same time? It's probably because you're a good Project Manager! Everyone knows you are capable and competent and your projects end up being successful. So your name is at the top of the list when the question is asked "who should this project be assigned to?"
Managing multiple projects requires multiple resources. One of the worst things you can do is to share your dedicated resources with other Project Managers on other projects.
If you only manage a few projects, then you probably know every task, contingency plan, risk, and detail for each project. But if you're managing lots of projects, then those days are long gone. You will need to delegate these details to someone else on each project team, to keep track of.
A common thing people forget when managing lots of projects is stakeholder expectation management. You need to keep everyone informed of the status of your projects and up-to-date with what's happening at all times. It will make your job much easier.