"Get out of your own head"

More context would help.

I interpret it to mean, Stop looking at things from such a self-centered point of view. Look at the whole picture. It's not all about you.

For instance, if one were continually upset by minor rudeness from others, a way to get out of your own head is to stop focusing on your own hurt feelings but instead to consider what difficulties the other person struggles with, which will help you to overlook the petty rudeness and instead have compassion for the other person, even thinking of ways to encourage them and/or lessen their burdens.

The way I have heard this phrase being used, it was meant to say "stop thinking/worrying too much about a particular thing, or about things in general", "get over (thinking about) something", or "enough introspection already, now go out and play".

When someone thinks too much 'in their own head' about an issue/problem/injustice/etc. they can work up a completely fantastical argument about how everyone is 'wrong' and they are 'right'.

And all of the distortion is 'in their head', it's not real life. A person being told to 'get out of their own head' is being told to talk with other people to get other perspectives on an issue.

I am not familiar with the "self-centered" meaning in the other answers. After some google searching and reading to find more usage context "in the wild" (eg this forbes article). I feel confident the most common usage of "in your own head" is effectively an antonym of mindfulness.

To be mindful is to be "in the moment" - focused on sensory perception and momentary feeling.

In contrast: to be "in your head" is to be focused on inner thoughts, sometimes to the point of neglecting your momentary senses and feelings.

Someone "in their own head" too much is walking around in a daze thinking about things far away in time, space, or reality. My understanding is that the content of those thoughts is irrelevant. One can be equally "in their own head" thinking about worries, fantasies, fickle injustices, the future, or the past.