Completely agree with above poster, some of the judgemental comments are disgusting. I know full well I’m going to be attacked over this but –
I think deep down you know what you want and need to do or else you wouldn’t be asking, you stated several times in your original post how much your life is going to change and how much is going to be taken away from you, children, travelling, a life with an equal partner?
I feel terrible for your Fiance, but feel that he’s got his head in the sand over his illness and what it will mean for you if you stay with him, you’re too young to be confined to a life of being his carer, and never being able to fulfill any of your hopes and dreams. Yes it’s horribly unfair what’s happening to him, of course it is. He’s going to miss out on so much, BUT does that mean you also need to? I can’t help but think that if he truly appreciated the sacrifice(and yes, it IS a sacrifice)you’re willing to make for him by marrying him, that he would try to spread his care and lessen the burden on you and show you that it will still be possible to have a normal life with him? We would all like to think that our other halves would look after us in sickness and health in an ideal world, but that’s not real life, the bottom line is – are you willing to sacrifice your quality of life to spend it caring for your FI? Don’t marry him unless you are completely aware and accepting of what this will entail, it’s not fair on him.
If you were older and had the life experiences you wanted and were already married then of COURSE it would be different and the right thing to do would be to care for him, but you’re young, and not married and you have a choice.
I might sound unsympathetic to those who are bashing the OP, I’m really not, life can be so cruel but if my partner was in the OP’s Fiance position then I know he wouldn’t want me to stay, not as his lover/wife anyway if it meant that our relationship would become more carer/friend, and all my plans of travelling, children, living life to the full were taken away, and I also wouldn’t want my Fiance to stay with me if the roles were reversed because I love him too much to see him throw his dreams away to look after me when I couldn’t be the woman he knew and loved.
Unless you and your Fiance can make serious changes with regards to his care which would still let you lead a relatively normal life then I think you need to stop this before you end up 10 years down the line in an unfulfilled marriage based on guilt, resentment and anger. I agree with solo counselling, and reaching out to his family and yours, you are capable of getting through this IF that’s what you want, you need to be completely honest with yourself and know that you are not a bad person for thinking about your own needs over someone elses, you have one life.