I'm sure it happens to most people. You find that when you're alone you can play naturally and just be in the moment, but when you know someone else is listening/watching, you feel like you have to put up some kind of performance (for fear of judgement, to hide) and can't really concentrate on the music. What are some tips to getting over this? Thank you!
Playing in front of an audience is a skill in and of itself. Like any other skill it needs to be practiced.
Pretending to be in front of an audience when practicing is one way to do it (though pretending only goes so far).
Setting up your practice space to be more similar to a performance in any way will also help. If that means playing through speakers, setting up monitors, facing a different direction (or if it is a band setting, having members of the band stand as they would on stage, not facing each other), changing the lighting, what you wear, etc can all help.
Doing a preview performance will also help. Find yourself a small audience, even if it is one person you are close to and play for them.
Record yourself, or even better video tape yourself. This provides GREAT feedback when you review it and knowing you are being "watched" may give you some of that performance vibe.
Also change your mindset. If when practicing you stop if make a mistake and start over, don't do that. Make a set list and go top to bottom, no stops. This helps you also work on your between song banter, transitions between songs, set length, and figure out when you need to take a break, tune, etc.
Lastly, perform a lot. Some things just can't be replicated. When my band was starting out we played a lot of shows where we were playing for the bartender, bouncer, and the bands we were opening for, but it really help us get ready for playing in front of larger (or any) audiences. If you need to get ready for a big event and don't have the opportunity to do this, maybe try doing an open mic night, play a party with only close friends, or something similar.
And lastly, before going on stage make sure you can play your music WITHOUT thinking about it (like topo morto says above). If need to think too much you may need to practice the pieces a bit more before going in front of an audience. Practice until you are free to just get into the performance and really rock.
Play in front of an audience. A lot. Start with the family pet and work up.
Close your eyes, if you can do that without it affecting your playing. There's a reason why so many vocalists close their eyes when they step up to the mic. If you can't close your eyes, then pick a spot on the back wall just above your audience's head and look at that when you look up from your instrument.
The most important thing is to stop caring what other people think. Yeah, I know, easier said than done. But for peak performance it is extremely important that you develop a growth (learning) mindset rather than a performance (fixed) mindset.
There is a lot of research on the importance of this for developing skills. The reason why the fixed mindset is often called "performance mindset" is that the focus is on looking good. The growth mindset is often called "learning mindset" because the focus is on deliberate and focused practice.
Do not talk about giftedness, inborn talents! One can name great men of all kinds who were very little gifted. They acquired greatness, became 'geniuses' (as we put it), through qualities the lack of which no one who knew what they were would boast of: they all possessed that seriousness of the efficient workman which first learns to construct the parts properly before it ventures to fashion a great whole; they allowed themselves time for it, because they took more pleasure in making the little, secondary things well than in the effect of a dazzling whole.
― Friedrich Nietzsche, "Human, All Too Human"
I seem to have the ability to at least partially block out my audience when I need to, and focus more on the band members, who I have rehearsed with and are comfortable with, I know the audience is out there, but it's not the audience I came to see. It's the music that I came play, music that I enjoy playing even without an audience. I'm not real outgoing and social, but others in the band are and they can take up the slack for me if I need them too. I am just focused on enjoying the playing of the music. It helps to know that that's why the audience came to see the performance in the first place.
Something else that sometimes happens is your performance takes on a kind of magical feel and that experience can keep you going for the rest of your life. I'm assuming by your question that you have not yet had such an experience.