[ar81]

Feeling jealous must have been caused by an experience in your childhood.
Were you detached from your mom in your first years of life? Were you displaced by a younger brother? Perhaps were you away from your mother for some time when you were a kid?

In adult life you can't control someone 24 hours a day.
If that person loves you, that person will stay.
If love becomes poisoned by ugly feelings, it is better fro both to leave.
My bet is that you are so jealous that you are incapable of loving your fiancee.

Fear of detachment is likely to detach you.
Unless you learn to separate that early childhood experience and your current reality, your relationship will certainly have no future.

Once you do not fear detachment, you have freedom and then you can love and improve your life and the life of your couple.

Do you need because you love, or do you love because you need?
Marriage is not the solution to your insecurity.
It is not the solution to your unhappiness.
If you are not happy and you fear, you won't be able to grow and your couple will not be able to grow either, and that's the best formula for divorce.
If you have a fiancee, you have little time to defeat your irrational fears.

"I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain."
Bene Gesserit Litany Against Fear. - Dune, by Frank Herbert

If the relationship ends (which is possible like anything in life) you must embrace it just like you would embrace relationship.
Certainly you need to separate that early childhood feeling from your reality, and that means to learn about being detached.
Attaching is a good formula for suffering. It is not love.
Unless you learn detachment, you won't be able to truly love.