I don't know where to start but here goes: 

I have a coworker who I've gotten to know quite well - We've had some deep conversations, told each other some really personal things, and overall I think we're good friends. She's admitted to me that she has some quite intense anxiety in her day to day.  I think it's crippling for her in a lot of ways - she's afraid to ask people for things (even people she's known for years, for example other coworkers, and she doesn't like confrontation of any sort and sort of shuts down when things get anywhere near any sort of discourse.  

I understand her in a lot of ways, and I have the utmost respect for the strength it must take her to get through each day.

That being said, I think I'm having a profoundly negative effect on her mental health and that I am exacerbating her anxiety.  We spend a lot of time together (same commute to and from work) and we talk a lot, but it seems to me like she's becoming anxious when we're around each other.  I think I may have crossed a line somewhere (I'm not sure where) but trying to talk to her and try to ask her what specifically is on her mind just leads to her tensing up and shutting down.   That being said, we still talk every day, albeit short conversations and never anything of substance.  I'm never **ever** going to directly confront her about this because I know how horrible that would be for her. 

I don't know what to do since I see her every day.  What I want to say is "I don't like how I make you feel.  I think we should just not talk unless it's work related from now on because I can tell I'm having a negative effect on your mental health and your anxiety is going through the roof when I'm around.  I care about you and I don't want to make you feel that way".

I feel like actions speak louder than words with her though and that maybe I should just leave her alone and leave it at that.  I can say with 100% certainty that she'll go on with things and never ask me why I've been avoiding her.  Our jobs don't cross paths either, we just happen to work in the same building, so that's a plus.

Can I please get some advice?  I don't have a lot of experience here and I've never met anyone like her before, so I'm just trying to do the right thing for her here given that I think I'm negatively affecting her mental health.