I think these are all open to so many views that it renders the terms totally subjective to the person using them.
I have friends who call themselves lovers, but they have been monogomous together for years.
I have friends who call each other girlfriends, but been together for years.
i have friends who say partner, when they have just began dating.
maybe it’s more about the agreement in the relationship as opposed to what word to use to call it?
for me, lover can be a whole host of relationships (friends with benefits To serious relationships)
partner for me rings more serious than gf.