It's not only wrong, but it's also toxic and hurtful to say "You can’t love someone until you learn to love yourself" for multiple reasons: It promotes neoliberal self-responsibility, consumerism, patriarchy, and ableism. In one of those end-of-the-year reviews, I heard someone say that they learned in 2023 that they first need to love themselves to love someone else. I decided to finally write something about this, as it made me angry.
Capitalism and patriarchy tell us that we have to fit certain beauty standards and gender norms. That we have to buy products and services to improve our physical and mental state to fit those norms and be loveable. This can hurt the self-esteem and self-love of those who don't fit the norms. Telling them that they are thus not ready for or deserving of love is promoting consumerism and super mean. Let's rather do away with those toxic beauty and gender norms (and capitalism).
Also, the idea that everyone is responsible for their own mental well-being is neo-liberal propaganda in the service of consumerism, telling you that you just need to work harder on yourself and buy some self-help products to fix yourself and live the dream. Most humans thrive better in environments of cooperation and support. While you should not depend on others to fix your well-being, it's a good idea to support each other in doing so.
Increasing existential insecurities created by the climate crisis, social injustices, and the rise of fascism, can have an impact on the mental health of many of us. This can lead to depression, despair about the own situation, and thus often reduced self-love. Does that mean that we can't love someone else? No. It doesn't make any sense to just ignore the world around us to appear happy. Let's rather face these issues of the world, society, and ourselves together, in solidarity.
Some mental issues can make it very hard for people to fully love themselves. Telling them they can't love someone else is ableist and can further worsen their self-worth. There is not one way to love someone anyway and everyone should be allowed to express love in their way without people telling them that this would not be the right way. Also, people on the autistic or asexual spectrum will have ways to show love that don't fit the norms. And that's okay. What the media tells us about happy romantic relationships is toxic anyway. Everyone has their own ways of expressing love and their own needs of what they would like to receive as love.
No matter if the relationship involves neurodivergent or only neurotypical people, no matter if one of them has to deal with psychological issues, e.g. rooted in childhood or past relationships, it's good to be aware of the basic psychological effects, of each other's needs and wants, and - if possible - talk about them. This is stuff that should be taught in schools.
It might be true that some people who currently deal with some psychological issues, don't have as much capacity to give attention and love as others. They are still able to love someone in their way, depending on their current abilities. They still deserve love. They should not expect relationship partners to "fix" them, but they also don't need to "fix" themselves before entering a relationship. It's okay for partners to support each other in consensual ways that keep the needs of both in mind if it does not create non-consensual dependencies on each other. It's okay to improve self-love and mental health while being in a relationship.