Thank you everyone for your replies and advice and experiences.
The thing is, I don't have many people on my Facebook to begin with, I have very secure privacy settings (that's how I've always been anyway over the internet), and I have a great support system. Some of my friends, and most of my family connections know to some extent what I'm going through. My boyfriend, parents, brother, brother "in law", father and mother "in law" as well as 2 friends know my exact diagnosis, while a chunk of my family and friends know I have some anxiety issues and pain issues resulting from a trauma (though they don't know an exact diagnosis or anything like that), and so far everyone has been very supportive. (also, I want to add that I say in laws in quotation marks because they are my family, even though I'm not married to my boyfriend, they are blood to my daughter, and I feel just as much family with them as with my own family)
My mom is one of my biggest supporters, even offering to bring my daughter to functions/birthday parties/activities whatever else on days when I'm not mentally feeling up to it.
With that all said, if I removed the people I could remove from my Facebook (old friends that weren't really close then, and don't talk now for example), I might feel more secure. I've been in the process of doing so.
I can't overly pin point the exact reason why I feel this overwhelming doom feeling if I were to post something reaching out and opening up. I have a few assumptions, but can't tell for sure.
I do agree, people who open up are extremely brave. Maybe that's it, maybe I'm not as brave as I wish I was and that's all that's stopping me. I don't know.
Again, thank you all for the replies and such, it means a lot!