Since charity is a kind of friendship, as stated above (II-II:23:1), we may consider charity from two standpoints: first, under the general notion of friendship, and in this way we must hold that, properly speaking, a man is not a friend to himself, but something more than a friend, since friendship implies union, for Dionysius says (Div. Nom. iv) that “love is a unitive force,” whereas a man is one with himself which is more than being united to another. Hence, just as unity is the principle of union, so the love with which a man loves himself is the form and root of friendship. For if we have friendship with others it is because we do unto them as we do unto ourselves, hence we read in Ethic. ix, 4,8, that “the origin of friendly relations with others lies in our relations to ourselves.”
St Thomas Aquinas, ST II:II q25 a4
I love this passage from the Summa. It’s so deep and affirming, and places proper love of oneself in such a high regard.
“Love is a unitive force.” A lot of Catholics like to always define love as “willing the good of the other”, which is a great definition in its own place, but seems to be used too often to suck the beauty and joy out of love. No, love is a force, a force that unites, the force that takes two and makes them one. This is also implies that our love of God is necessarily directed towards our divinization (or “theosis” as the Greeks call it).
In a sense, “self love” is an inappropriate term, because love is a unitive force between two people. But a man is “one with himself” which is something more than unity or friendship, more than love as we generally use the word. It’s something more, not less.
To love yourself is to be one with yourself, and it’s only from this unity that love of others is possible at all. This is the principle, the form, and the root of all other friendships. We can be united to others only insofar as we are one with ourselves. This is a truth that’s confirmed in all my experience: the best and most reliable friends are those who love themselves best, knowing their own worth and goodness.
This idea of self love as oneness leads to an interesting link between self love and integrity, which is also a matter of being one, whole, integrated. If you love yourself, you won’t deceive or betray yourself. You’ll be true to your own values.
So, how do you actually love yourself?
Some people really struggle with this. I think it’s something most of us are ignorant of in our society, and if anything it’s even worse among Christians, since we place so much importance on sacrifice and dying to self and our own unworthiness of salvation. But God made us good, and it is good to love what is good. God loves us, and it is good to love what God loves. And it is good to love those who are closest to us, such as our friends and family, and no one (besides God) is closer to a man than himself. Accepting this is a good start.
Then, treat yourself like you would treat a good friend in your situation, or like a good friend would treat you. Or as Jordan Peterson puts it in ’12 Rules for Life’, “Treat yourself like someone you are responsible for helping” (rule 2). It takes a little time and imagination, but after a while it grows more and more habitual to be good to yourself. It’s not merely indulging yourself, and it’s not just putting in the work to improve. It’s caring for your good as a whole being. You need exercise and sustenance and rest and recreation [what a beautiful word].
As noted above, integrity is critical to self love. You have to recognise your values and live according to them. You have to be honest with yourself and with others, because we can only be one with ourselves to the extent that we hold to the truth of who and what we are.
Going to confession is a good exercise in this honesty and integrity, as is doing penance, because it means facing up to ourselves including our failures, and then working to make things right. It might sound paradoxical to say penance is an expression of self love, but there it is.
Another important part of self love is surrounding yourself with love. Make the effort to stay in touch with family and friends, and to make new ones too. Frequent the sacraments. Read scripture and good books. Make time to pray. Watch wholesome TV (‘Queer Eye’ springs to mind). Spend time in nature (it’s strange to say, but there’s a lot of love in nature).

That’s all I’ve got for now! God bless!