The hard moments are supposed to be hard. Let them nourish you. Feel them because they are real and they hurt, and because feeling and hurting are part of you, part of growing, part of being human.
You will have something else to worry about tomorrow.
Nothing makes sense until you've put it on paper.
Don't be constrained by other people's dogma. Don't be constrained by your own dogma, either, but if you are, it's ok to reevaluate the things you once held as absolute truths.
Your expectations of yourself are excessive beyond measure. Your instinct may be to try to fix this, but don't waste your time. You will always hold yourself to insurmountable expectations, even if you realize their pragmatic impossibilities.
Being small is comforting. It's ok to want to contain yourself. Being guarded is not the same as being impenetrable.
There's nothing wrong with you.
You're not broken.
A relationship doesn't have to be romantic in order for it to matter. A relationship is a living bond between two living things. Your relationships with your friends are worth fighting for. Your relationship with your body is worth fighting for. Just because they aren't romantic doesn't mean they're not real.
Relationships are (or have the potential to be) ok. We're supposed to want other people, to need them in ways we can't understand or qualify. Vulnerability isn't a four letter word.
Let other people in.
You can't change your biology or evolution or science. You can't change your thoughts either. The only thing you can do is shake hands with them. Watch them. Acknowledge their space in your life. And then let them go.
Mindfulness is a lot like cronuts.