Staying in a relationship where you are not well appreciated is one of the worst things that can happen. Likewise, begging your partner to love you when he/she has decided to stop is unimaginable.
If your fiancé/fiancée would not willingly stay in your life, the last thing you should do is beg him/her to stay.
If you beg them to stay today (while you are still pretty, handsome and bubbly) and you eventually get married, what gives you the assurance that they won't decide to opt out again when you have become pre-occupied with other things like; career, child-bearing, housekeeping etc.?
We all know that a broken relationship is better than a broken marriage. Would you not rather let them go now than after you've had children or after you have built your life all around them?
A person who is worth walking down the aisle with is someone who wants to get married as if their life depends on it. Keep this at the back of your mind and let it guide you in deciding who to spend the rest of your life with.
Remember, "If you have to beg someone to be in your life, they don't belong there".