I need advice about how to open up to people?

This is a little personal, but here goes:
I have a really good friend, but I can’t call her very close… she reached out one day and told me that she was there for whatever I needed to talk about. I’ve been going through some things, trying to work through them, and she could tell. And although I really wanted to confide in her, it was like my tongue wouldn’t work…
I don’t have anyone else, either, to turn to…
Meanwhile she’s told me a ton of things she won’t tell anyone else, trusts me, is still there for me, but I can’t seem to ask her for help… Why might that be? What can I do to get the guts to take the hand she offered? Should I?