Why would you want to share your thoughts and projects here or anywhere on the internet? What are the benefits and downsides?

The largest benefit of sharing is putting your ideas in words. And not just any words, but words that other people who don't know you could understand, which makes it more objective. This helps you think things through better. It also helps you see what you do with your life better, so you can understand yourself and your activities better.

The second best benefit is feedback. People may have suggestions, see things that won't work, or not understand something you say and let you know.

A third benefit is helping others. People may benefit from seeing what you're doing. And, similarly, you can benefit from seeing what they're doing. You may learn from others, inspire each other, or avoid repeating the mistakes that others make.

My focus above was on sharing what you're doing. These benefits also apply some to sharing your comments on what others are doing or to having a discussion. That's also putting your thoughts into words, and you can get feedback that way, and help or be helped by others. Collaboration is a two-way activity. If you want feedback from others, you should also be prepared to give it to others (or pay for it instead).

Giving feedback to others will help you better understand what giving feedback is like, which helps you share your own stuff better. You'll learn about what makes it easier or harder to give good feedback, and then you can handle that better when sharing your own activities.

A risk of sharing is you may be more judgmental with yourself when you imagine, in your head, how others might be judging you (usually they don't care about you that much, and aren't paying that much attention to you, so you're judging yourself way more than they do).

Another risk is that you could do whatever other people suggest instead of making your own decisions. You could be too easily influenced. It's important to view what other people say as potentially-useful brainstorming, suggestions, and options that you can use as you see fit, not like something your teacher or boss told you.

You could also share stuff that violates someone's privacy. Stop and consider carefully before sharing stuff that violates our society's conventional privacy norms, especially about other people.

You could also develop a bad reputation. People worry about looking dumb, but I don't think that's a major concern. Generally, if you try to learn or accomplish stuff, most people will be impressed that you're braver than them and taking more initiative than them. However, if you get in fights with people or act like a jerk, people won't like you.