Caring less about what other people think has two aspects, thinking better of yourself and worrying less about others. The first is relatively safe. The second is riskier. First, the moment you assert boundaries, everyone who enjoyed treating you as a doormat will object, sometimes violently. Second, if you take it too far, you can turn into a jerk, which is rarely the goal. With that in mind ...
Knowing yourself is the first step to caring less about what people think. Until you know that, you can't tell the difference between what you think and what they think, which is a horrible problem to have. Get to know yourself. Here are some questions that may help.
Understand self-worth, which is different from self-esteem. In essence, self-worth is about being a good person and knowing your strengths, while self-esteem is a good opinion of yourself (justified or not). There are many ways to build your self-worth, such as developing virtues and skills. Realize that this is a lifelong practice, not a checklist to get through. When you work on yourself, you create a strong foundation to think well of yourself rather than looking outside for comparisons or approval.
Learn about self-compassion and practice it. Here are some exercises. This will help you avoid beating up on yourself or making negative comparisons to other people.
Personal boundaries come in various types. They should be semi-permeable, not loose or rigid. You need these to prevent other people's problems from becoming your problems. Here are some worksheets. There are also ways to form a mental shield to protect yourself from what other people think. If you feel like folks are flinging crap at you, they probably are, and this is how you can block it.
Assertiveness is a skill you will need to set and maintain boundaries, because other people will try to manipulate you. It applies to various situations. Learn how to be assertive and ways to say no. Just remember, don't be a dick. You can be both tactful and assertive.
Pay less attention to what other people think. You will learn some interesting things this way. Focus in other directions.
You need to set goals to give yourself something to focus on besides other people. Learn how to set goals. Some people like the SMART goal approach. However, there are many options; here are some goal-setting worksheets. For fannish activity in particular, consider creative bingo. A community like allbingo will give you prompts, and you can decide on goals like "I will write 5 short fanfics to make bingo."
Finally, build a good support network. Good friends will shore you up; they'll say "Go you!" when you succeed and "That sucks, but you can try again tomorrow" when you fail. Bad friends will tear you down. So one oblique method of not worrying what people think is to surround yourself with people who don't say worrisome things to you. Here are some ways to build your support network and a worksheet.