Hi Amy,
Think it's clear from people's comments that a)you can, but b)you might gain something from involving others.
For experiemental films (your first films) it's perfectly okay for you to practice by yourself till you are comfortable enough to do it with others. Using inanimate objects or animation / drawings might make it easier than trying to shoot & act. Being Director on a set can be quite intimidating for anyone the first time, and if you're a loner, I can understand you might not feel comfortable making mistakes publicly. (Between you and me, my first short film was at uni, and I did exactly what you're proposing - I used a camera, a tripod, and myself. It wasn't a very good film, but sometimes it's what you've got to do).
When you feel more comfortable, if you're serious about being a filmmaker, you will need to start working with other people. The film industry is a "networking" industry, which means you need to meet and mingle with a lot of people. This is partly to source people to work on your films, and also because it is through networks that opportunities come. Someone likes you, offers you a job, or invests in your film, or agrees to act in it at budget rates becuase you aren't fully financed, etc. You also have to pick up the phone and cold call people who you've never met to see if they'll come work with you.
But you don't have to jump into that networking stuff yet. When I was your age I was the same, but now (I'm 31) no one would guess that I used to be shy. Part of growing up is about getting over your inhibitions, getting better at doing things you don't feel comfortable doing (till you've done it so many times you do it without thought), and also generally getting to like yourself better as you start to recognise your strengths and unique talents.
And, while you say you spend some lunches just walking around by yourself, I suspect you're doing more than that: you're observing others. You're overhearing their conversations, imagining what they're thinking, possibly creating imaginary dialogue or situations with them in your mind. That will help you to be an excellent filmmaker.
Good luck
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For case studies and advice on innovative ways that indie filmmakers plan, finance, distribute, build audience and monetise their projects, stop by my blog - and contact me if you want your project profiled. http://yetanotherstrugglingwriter.blogspot.com
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For case studies and advice on innovative ways that indie filmmakers plan, finance, distribute, build audience and monetise their projects, stop by my blog - and contact me if you want your project profiled. http://yetanotherstrugglingwriter.blogspot.com