The distinction between “happy about” and “happy with” is a subtle one, and sometimes the two expressions are quite interchangeable. As a rough guide, here’s how you can recognize when to use which:
be happy about = be pleased by; be glad that something happened
For example, if you are satisfied with your car, you could say:
and if you are satisfied with your own actions, you can say:
On the other hand, if you want to express that someone is (not) pleased by some news, you can use “happy about”:
Nevertheless, you will hear some native speakers use the two expressions quite interchangeably, so even if you confuse them, you will likely be understood. If you follow the usage guide outlined above, you also reduce the risk of sounding unnatural.