Real quick I want to add this before I have to retire for the evening.
This is a subject close to my heart because I've learned a lot about it and me in the past couple years. I USED to have a hard time opening up because to do that also means being invested in others. It's easier not to be invested sometimes. Takes less time and you can be more selfish (fun, lol).
Now, I give friends and new people I meet subtle little clues into my life's challenges, my past, my dreams. Many times, this is all it takes for that person to open up to me a little bit too, sometimes a LOT! Then we have more in common and the more I share the more they open up the flood gates. It creates deeper relationships rather than surfacy "top this" relationships. I HATE those. They suck me dry even more than being invested.
Edited to add: I've learned the hard way to only give people information they can "handle" about me. If not, sometimes it can come back to bite you in the butt. More on that later too. I have experience there sadly.