[¯|_(ツ)_|¯]

I think that it would be okay to tell her right away. You can start out by saying that you have been going through a lot lately, and you decided that you need some help and support for it, and then begin discussing what is going on.

When I talked to my guidance counselor, I handed her a letter the first time. I did this for a few reasons. One, I knew that if I handed her a letter I wouldn't be able to back out. Nerves or interruptions wouldn't get in the way of me saying what I needed to. Also, I knew that I wouldn't forget to tell her anything by accident, either! So, letters are an option.

Even if you don't give her a letter, you can always make an outline of the key points you wish to tell her, so that way you don't forget anything.