Give her the gift of missing you
My 2 cent.
Been on all sides of this, even read a big book on it to get a good understanding. First, I know how it feels, i've been in a long relationship break up and dam it's tough. I am gonna try and write this without chucking generic terms at you, as you can read them online anywhere, like Reddit.
DP - Yep dating can be hard, sometimes you have to learn to love with your head than your feeling when you are numb and I know from experience I've lost many friends due to another illness at the time I have as I just too ill to even function, being in a relationship is hard work and it's more than being a therapist or feeling like you've got one, that's why they exist. Sometimes you got to be in the right place for things to happen, it's like how many opportunities (I dunno how old you are) have you had that you ever thought "man, if that had came 1 year ago I wouldn't have been ready"...
If you want to be the 'nice guy' you will become the support, a friend, a platonic relationship, this lows attraction, could go into detail, but i'd advice reading or audio book of
https://www.amazon.co.uk/Winning-Heart-Woman-Your-Dreams/dp/1411673360, Forget the winning the part of your dreams blah blah, it helps you understand the roles in relationships and is one of the best books i've ever read, even talks about when to walk away, it's helped me recently even with friends who are guys, the guy is a bit of a player, but his understanding is great.
You have to understand a relationship works two ways, honestly what you two want are different things, this can't really be. Not currently anyway.
So either be a lovely friend, therapist and accept it and understand that if you do this, that won't change. You are friends now. I'd personally say no, worst thing to do. You will be stuck thinking about a girl who if needs you, she will call you, but you'll never feel her love. She will make you constantly just feel rejected. I've rejected people who were my favourite people, illness, or even crushes, one wants to date, one doesn't, been on both sides, it's back to the, you want different things thing..
If you only see this person as someone you want to be with, Chicane's quote above. You won't FRIEND HER into wanting you, if anything you being there all the time will do the opposite. Again, lets be honest, you didn't write this post because you wanted to be her friend....you wrote this because you want to win her back.
You can't change her mind, you can however give her space, people want what's not available to them, what they miss, it will also give her a chance to focus on her illness, her health and get things straight in her life, sometimes we all need that. It takes two, right now she needs space, you'll just push her away if you do anything else and you need to focus on yourself, not spend the next x amount of time trying to impress or 'win' her over or change her mind.
"What you fear you attract, and what you look at disappears".
I'm not saying be a cold dick btw, if she needs help answer the call, but don't text her, i'd not reach out at all, she walked away.
My takeaways :
Give her space and let HER come to YOU, she makes the effort, she comes to you, she left, you didn't shift. You are grounded and have more in the universe to offer than advice.
Unless you just want to be her friend and that's it, you can't be her friend try and go under the radar wanting to be her boyfriend the whole time, big red buzzer.
Focus on yourself
If she does talk to you, you don't want to just be a friend, keep it short, ask if she wants to see you, if you don't make a romantic plan, keep the conversation short and get off the phone, don't think talking for 2 hours and catching up on the phone is anything but being friends. Don't act like a clingy girl. This won't be ease, but it's a sure way as the nice guy to be her best frewwwend.
Realise how you feel, If you need to, even for a short time, try a therapist to talk about how you feel, monitor how your depression and anxiety is, go to the gym or for walks and remember to look after yourself.
Break up's are some of the hardest things, getting out the house and just hanging with friends, taking your mind of stuff is one of the best things you can do. Often break up's can be sore on your confidence, that's why a lot talk about going to the gym, doing things they are passionate about, at the start it will be hard to find anything, probably be on your mind a lot, it does suck but that's natural and part of the process my friend.