In love with ex. She has cancer.

Hi.
Not really sure what I want out of this but maybe just writing it down will help?
Three years ago, I met the most amazing girl. The first moment I laid eyes on her, I thought, wow!
We dated, became official, fell in love and I thought that it could last forever. Fast forward 18 months and we agreed to split. She had kids, the most incredible kids in the world, I loved them so much but I wanted my own. For perfectly understandable reasons she didn't want any more and so to spare ourselves heartache down the line, we brought the relationship to an end..... Well, actually we continued to sleep together for a few months as neither really wanted to let go but eventually, we went our separate ways.
I missed her so much and when we happened to get back into contact last year, I was over the moon. With no desire to have children of my own anymore, I thought that maybe, we could fix it.
After a few lunches and a kiss here or there, I asked her but she explained that she needed to be on her own for a bit.
We continued to meet up and then, early this year, we ended up in bed. We agreed that sleeping together was not a bad idea as no one else was involved and we went on from there.
As time went by I realised that all I still wanted was her and that maybe, in time, she would feel the same.

Then came the biggest spanner I could have ever imagined, she found a lump in her breast.
A week down the line and she was diagnosed with cancer, facing a mastectomy and chemo.
There and then, I made my mind up, whatever it takes, I am going to be there for her. School runs, hair loss, anger, weakness, the lot. I cannot watch this from the outside, I have to be there, not just for her but for me too, I cannot be away from this.
We are in what you would describe as an open relationship, not that I've even considered sleeping with anyone else, I've not been interested in anyone else since we first "hooked back up" and also, if she made my dreams come true and told me that she wanted me back, I'd want to be able to honestly say that I've not slept with anyone else since she we started again.

Now the difficult stuff; I can handle all the shit that comes with cancer, she is the absolute most important thing but it is breaking my heart that I can't be more to her. I want to hold her at night when she can't sleep, I want to be able to take everything off her shoulders when she needs to but she is holding me at arms length and I hate that. I want to be more than just a friend and sex buddy.
I understand that she is scared and needs to concentrate, solely on getting better and is probably trying to protect me but not being there full time is starting to break me.
We find out soon how far advanced the cancer is, I can handle anything but stage 4, she feels the same. Stage 4 would destroy me.

I guess the question is, what do I do?

Regardless of anything, there is no walking away. She has the hardest year ahead of her and I will not let her go through it alone, I love her, unconditionally but whilst I understand how tough this is on her, the worry that she'll push me away is hurting, so much. I am so scared about her upcoming results, maybe it is that that makes the relationship situation worst?
Like I said, I don't know what answer I want from this, maybe just writing it down will help?
I've read a few threads on here and whilst I'd like honesty, please don't suggest that I am being selfish, yes I want her back but her health and wellbeing is far more important to me, she is the most important thing in all of this, not me.
Thank you.

I don't know who would suggest you are being selfish, and I would be rather miffed at anybody who would. Being selfish would be to expect her to focus all her attention on you instead of her health. Being selfish would be to ditch her in her time of need with little or no reason.

You are certainly NOT being selfish by being there for her. This is an extremely difficult time she is going through, and you are willing to be there for her and go through all of it with her. That is NOT selfish. Also helping your case, in my view, is that fact that it isn't like you suddenly learned she had cancer and THEN decided you want to be with her. You'd already sort of been leaning towards wanting to reconcile anyway when this terrible news broke.

Now, that said, there honestly isn't much you can or even need to do beyond what you are already doing now. The only thing I would really suggest is to talk about all of this with her. Tell her the things you've said to us here. That you want to be more for her, that you want to be there for her. I would say don't be too pushy, though. In other words, if she is iffy about letting ANYBODY (you or otherwise) get that close because of everything she is going through, don't push it too much. Just keep being there for her and hopefully she will let you in. However, that doesn't mean you can't at least try first.

Obviously she still cares about you at least in some way. For her to still be allowing you to be part of her life, she obviously still likes you. Maybe that can become something more. Or maybe she'll just feel too overwhelmed with everything to allow that right now. If that is the case, you can at least be there for her in any way she will let you, and hopefully that will grow in time.

Good luck to you either way. Good luck, as well, to her. I hope this turns out not to be serious, and is something she is able to defeat easily. It is a struggle, but these days more than ever, it is one people can win.