My fiance is pushing me away - do I stay?

Some context: we have been together for over 6 years, engaged, newly home owners and have a dog. After a crap start to the year, things started to pick up and we were (so I thought) in the best headspace we’ve been in throughout the relationship.

3 weeks before we were moving into our first home together, my fiance went quiet with me. This is normal and can last a day or two, I give them space and they usually come to me for help eventually.

This time was different. What I thought was just bad thoughts from mental health was actually my fiance dealing with thoughts regarding our relationship. Shortly after they asked me if I would consider opening the relationship and they miss the nervous and excitement feeling you get when you get to know someone new. I thought about it for a bit and my answer was no. They respected my decision but it seems to have pushed them into a depression

The way they described it to me was as if there was a self destruct button in their head that has went off. They are struggling to understand why they would want to open the relationship when they have a safe, comfortable and loving partner.

it’s been 2 months now and they are spiralling further and further down. They don’t do any of the good hobbies (running, gym, walking) we used to do together and instead are doing the old bad habits (drinking, vaping etc). They have found new friends and meet up with them for drinks whilst I’m at home with our dog. My thoughts go wild as I have lost all trust since they asked me to open. It feels like they are putting more effort into drinking and socialising than they are in to fixing our relationship.

It’s gotten to the point where I received a message saying

“You deserve to be with someone who is happy and can give you the world”

Today they spoke about breaking up with me and I honestly don’t know what to do. It hurts me to think that this is may be the end of our relationship.