Thanks for explaining that you are both in High School which makes the situation even more difficult for both of you. Since you are both just kids, it is obvious if she has serious issues it is way too much of a responsibility for her to unload them on you. And it is way more obvious if she has serious issues that she needs professional help.
All the more reason to set boundaries for yourself and offer to support her in her choice to find the help she needs. You can offer to go with her if that helps as well, even if it is to hold her hand while she makes a phone call to a help line to gather information. At least that would be a first step.