Why my best friend accidentally triggers my depression and anxiety, even though she didn't do anything? I get triggered just by thinking about her, or seeing her, why..?

First of all, it's good that you have an understanding that your friend is only triggering your depression and anxiety and not directly causing it. It is good that you are able to differentiate this. With this understanding , you can work on identifying your friend's actions , behaviours or even speech which triggers you off. From there , you can retrace why,how, when these triggers sets you off. When you settle your old unresolved issues then you can reboot your old thought pattern. Look at your friend as who she or he is. This friend is not similar to someone in your past. Avoid labeling, grouping or categorizing your friend with people you have met and left a negative impact on you. Be aware of your perception.

I don’t know the history between you and your best friend, but I am assuming your best friend may have been around during some rough moments in your life. Your best friend’s presence may remind you of your anxiety triggering history. Some people remind us of certain times in our life. Even if they were supportive, they can still become a trigger because they are a symbol of past pain. Especially if the best friend was the person you turned to for help or a listening ear. Also embarrassment is a possibility. If your best friend saw painful things happen to you, you could possibly be embarrassed because this person knows some of your darkest moments.

while i cannot tell you how to live your life or what goes through your own head, i can say that this might be something to work on with a therapist

Maybe you are an empath and you are sensitive to her energy. I have experienced that. I made someone that I barely knew cry from being close to me. She said she felt my sadness and that she just wanted to help me and make me feel better. The weird thing was, she war right. I was in a very bad place at that time and that was a particularly rough day for me. She felt it. With that being said- it’s a real thing and you may be experiencing the same thing. Just a thought.