I have experienced both losing a best friend and losing a partner. I would rather go through a breakup in a relationship. Losing a best friend is one of the hardest things and I'm gonna share as to why I think so.
The two of you did everything together. That person was your best friend. You lose that one person you can do literally anything with.
All of those meaningful conversations with your best friend can't happen anymore. All of those inside jokes are slowly forgotten and it's depressing. Also if you're going through something you can't reach out for help. You just have to deal with it on your own. Yes, in romantic relationships this is also a con, but for whatever reason these memories are more meaningful with your best friend.
After all this time of being able to go to your best friend for advice you can't do that anymore and it sucks. It's the worst thing ever when you need to share a secret but your best friend isn't there anymore.
Remember the inside jokes you had with your best friend? You two would laugh every time they were brought up? Well imagine the day where they lose their touch. This is what happens once you lose your best friend. The inside jokes are soiled and they become meaningless.
It sucks if you told your best friend everything. Now you can't share events happening in your life. The excitement to share your good news with your best friend comes and then you realize that person isn't your best friend anymore and you are left with no one to tell.
Have you ever lost your best friend? Which one was the hardest for you: losing your friend or your relationship?