I am the person in our relationship with the chronic illness. It is difficult place to be in. I have other outlets then my partner. For me, I have feelings of fear that I am not enough and am going to be left behind for the healthly person.
I have spoken about this and gotten reassurance in this regard.
I think the key here is communication. Communicate your fears and feelings to your fiance and to your new partner.
I feel a great amount of empathy for you,