How do people handle insecurity, impostor syndrome, and other mental blocks?

I empathize with you as I struggle with this myself, and have since I was a kid. The honest answer here is that you have to attack it head on, taking it very seriously - it's not going to just 'go away'.

I strongly suggest that you make the investment in therapy for this - it goes a long way, and the earlier you start the better off you are. Talking through your challenges is very powerful, and can help you frame what is going on. It also can help you get to the core of the reasons you feel the way they do - they rarely, if ever, are simple. 

Outside of that, some thoughts that have helped me:

• Delete social media, or limit it as much as you can. I recommend deleting it - but I'll leave that up to you. It doesn't help. 
• Make a plan for yourself. Start short term - like weeks or months - and then go out long term. Evaluate what you want, what makes you happy or drives you and align it to that. Set benchmarks and check ins with that plan. This is so important, because it's so easy to get hung up on feeling left behind financially when, really, there are tradeoffs to everything. 
• Read more and start writing about it. Literally - write about what you read. Could be here on this forum, could be on a blog or could just be in a random notebook you have. What's important about this is that you develop your own thoughts about things and your own views on what you read. I've found that a powerful way to build that confidence in yourself that you know what you are talking about. We so readily want the 'cliff notes' or the 'right understanding' when the reality is so much more complex. 
• Find things that you hate, scare you or make you nervous and do them. Doesn't really matter what - just that you take action and do them. Doing the hard things in life makes the rest that much easier. 
• Seek out a mentor for your career, to help give you that long term perspective on the professional side of your life. Having someone who can help guide you is so important, and honestly overlooked in most cases. 
• Travel more. It may seem an odd one, but if you haven't go as many places as you can. Discovering other people, cultures and lifestyles really changes how you view yourself in the world. Experience things for yourself, and not through others. 
• Last one is exercise, both indoors and outdoors. I think the outdoor component is really important to this - get outside as much as you can. 

There's a million ways to do this, but I'd emphasize that much of the above has worked for me and, to be sure, it's still a work in progress. It's hard to tell someone, but age does some of this work for you. It provides perspective, experience and really a comfort level with yourself that at 21 I would have laughed at. You'll get there, keep working on it.