Maybe not constant happiness- no. I find these sorts of presumptions weird though. It's like telling someone laughing uncontrollably at a joke that it really isn't funny. It's like telling someone who adores the taste of chocolate that in fact- they are mistaken. How can one human decide what another human is feeling or, what they value?!! It's just as bad as someone saying- you shouldn't be unhappy. Nothing is worth you wanting to kill yourself over. It's down to our individual experience and perception of this world- surely?
Of course- you could of course argue whether it's likely someone will be all that happy in this world. Even whether it's morally ok to feel happy to be part of a system that exploits others while we are being exploited.
Still- as an issolated emotion, I think it's presumptious to invalidate what someone is feeling. It's like saying- ok everyone- the colour blue doesn't exist anymore. Stop seeing blue everyone! And if you think you have- you're f*cking crazy! Personally, I find nihilists just as single minded and intolerant as pro-lifers. It's a point of view- not a reality that everyone shares.
Emotions aren't exactly right or wrong- they are a reaction to a stimulus. They're not like a maths equation. Imagine if we were tested on whether we found a joke funny or whether we liked a certain scent or colour. Maybe we are now though in a way! With all these wannabe TikTokers and YouTubers becoming famous by reacting to things! Influencers or whatever- incase you aren't sure what to like maybe.
I'm sorry about your partner though but again- I think it's weird to say to someone who is having or had a genuine relationship that made them happy that- actually, that was all a delussion. You weren't really happy- even then. I just think it's weird for them to think they know other people's minds.