Ending a Friendship

I’ve had this friend for over 8 years, but I have noticed that overtime and especially after college we really have nothing in common. I am at a pretty rigorous university and traveling abroad at the moment while she’s on a different path at a D2 school. It’s just a different life experience. This sounds mean but I just feel like sometimes with my high school friends I have to dumb myself down to have conversations and it feels really inauthentic, especially with her. On top of that she’s a big weed and nicotine smoker while I have quit both and was never really a regular smoker. There’s just so many differences and I honestly feel like shit after our recent hang outs, but I feel bad because she’s always had a lot of mental health issues. She still considers me one of her best friends even though she barely knows anything about me.

How do I go about this? On one hand I think it’s for the best for me to end the friendship, on another I feel it might be too harsh and I should stay from a safe distance and be there if she really needs me. What do you think? It’s been on my mind for this whole semester tbh.