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That must be quite frustrating and difficult for your self esteem.
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You may be right, but you might be putting your own insecurities on to the motivations of others. The world from other people's perspective don't always align with what we think. Could what you describe be as easily explained by something else, perhaps even your insecurity about it?
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Have you discussed this issue with friends / family and gotten their honest thoughts? Do they agree that what you say could pose a problem?
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Have you attempted to make yourself look older with makeup and such? Are the results the same with as without?
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Do you have male friends you can ask if you are attractive?
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Are there republican conventions nearby? If the men there show interest and say you're mature for your age, you may want to A. Run B. Conclude that your features may be a challenge.
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If men don't approach and talk to you, try approaching and talking to them. Showing you're interested and available is usually a turn on. Men are simple.
Best of luck.

If it's not because of that, then what should I do :(