I am so very, very sorry that you are in that situation. How heart breaking!

I think your distress is totally understandable given the unhappy circumstances.

It could be that you fiancé is suffering from clinical depression. That can sometimes require medical treatment to resolve. A person in the grips of depression does not always think rationally because of the brain pathology than can be involved in depression.

Is your fiancé in the care of a good doctor or psychotherapist?

I was always told to avoid making big decisions while under the influence of depression.

Perhaps I am wrong about what I have written. I am often wrong about things and am not myself a medical professional of any kind. I wish I could write more now but I am ill and cannot type for long.

Hopefully others here with more similar experiences will see your post and respond with things that are truly helpful. Sorry I cannot be helpful to you but I do want to say that my heart goes out to you.