I want to learn to put on my armor like everyone else. We're all taught to be ourselves and be selfless and soft and kind but I don't want to do that anymore. At least not at first.

Any advice would be appreciated. How do I become stronger so I don't get eaten alive?

There are different ways to become stronger. Your sense of strength may be different from someone else. SOmething i tend to do is im silent in public. Im a quiet guy till you get to know me and that soft and kind side, like you said, opens up because their is trust.
It's not for someone to tell you what your strength or 'armor' is. Itll come to you.

Now if people are ever treating you badly in any sort of way and thats what you mean by 'eaten alive' then stand up for yourself. Protect yourself and dont let people push you around.

My advice is to get a hobby and take up some form of exercise. It sounds like whatever has altered your prerogative is stress related. If you really want to be hardened, take up exercising and learn to detach yourself from your emotions. just train yourself in small ways not to be so emotional, and build up until you feel you are as strong as you want. Just know, the path you chose may be a lonely one. Good luck.