Hi, I'm Aliza. I'm a Breast Cancer patient. I'm also a Medical Librarian (retired) who does research for people here on this site and elsewhere. I'm engaged too. I became engaged last Spring and my fiance and I (we're both 54 [2nd marriages for both of us]) were supposed to get married last Fall. Problem - I got diagnosed with BC in August. My fiance is a widower whose 1st wife died from ALS. I had Stage I Breast Cancer, but I knew my fiance had gone through such an awful time with his wife that I told him to "run like XXX" when I got my dx. I understand exactly why your fiance did what he did as awful as you feel and as sad as he must feel.
My fiance wouldn't go. He stayed despite my telling him repeatedly to leave and finally I stopped. I had a mastectomy and he stayed. (I'm being treated at Sloan Kettering in NY where they do "instaneous reconstruction" so I have an expander in place to get my "breast" ready for an implant. I'm not flat-it kind of looks like a breast (sans nipple). My fiance is fine with that.
We both just read your very sad story and I told my fiance that if I'd had Stage IV breast cancer I'd have shoved him out the door (maybe paid for his train ticket on Amtrak online, printed and handed it to him) and my daughter and brother would have had to pitch in more. I wouldn't have wanted him to watch me die either. It's a horrible feeling of guilt that no one wants, to think that you're dying and you're making someone else suffer, especially since you weren't married for 20 years and didn't have kids. I'm not trying to upset you. The time element of my relationship is about the same as yours.
It's hard for me to offer you advice (you're not my daughter), but I think since you were your fiance's caregiver, I would recommend that you give CancerCare a call. The Social Workers there offer counseling only to Cancer patients and their Caregivers. You were his caregiver. I have a feeling that they'd bend the rules a bit considering what's happened to you.
You asked whether your fiance's "survival instinct" would kick in. I'm guessing you mean whether he'd fight harder. That's very hard to say, but signs from the time he told you to leave don't point to that. You'd need to verify what's happening with someone who's seen him recently.
The other thing I'd suggest is that you seek some counseling from a woman's center (at a university perhaps).
There are hard realities here - your fiance has the (legal) right to have his medical records be private from anyone he chooses including you and he has the right to restrict who can visit him as well. If you're worried about him, perhaps you can call a member of his family or a mutual friend to see how he's doing without upsetting him. If you're worried about him because of his depression, you can call the psychiatrist at the hospital where he's being treated and let her/him know what happened re your relationship and that your fiance has a history of depression and should be evaluated. You can always call a physician to tell them something about someone (your fiance may be mad as heck at you for doing this [it's possible the doctor will tell him that you phoned]) but the HEPA laws will prevent you from finding the outcome through the hospital or doctor. I'd recommend that you not use the hospital as a means to insinuate yourself back into his life.
Please realize that this is a very difficult situation for yourself and for your fiance as well. I think everyone deserves the right to choose how they want to die. I'm sure he loves you and I'm sure he wants to protect you and in an extension himself from thinking he made you go through a horrible experience.
Make no mistake, this is a horrible experience for you anyway (he won't see it like that [he can't be that clearheaded at this time]). I certainly understand that and hope you get whatever support you need to get through this.
Make sure you have good friends to lean on for support at this time and do make sure to get counseling! If I can be of help to you in any way, please feel free to message me or email me privately.
Wishing you the best in a difficult situation,