Oh that's great about therapy, give your friend a kick up the arse. It might be worth having a look at my post to @Friendlygingercat about getting a psychotherapist not a counsellor, and the type of psychotherapy that's probably best placed to help.
It's just occurred to me though - there is such a thing as existential psychotherapy. I know very little about it clinically, but it starts from precisely the philosophical point you are at. I don't think there are many in the UK but there are definitely some.
I do hear, though, that although there is an existential element to your feelings, it's also very much about the relationships you have with others and with yourself, so this might actually be a bit of a red herring (as might be my philosophical suggestions from earlier). Perhaps really all that is needed is some good therapy and possibly a diagnosis?
On another note - my life improved immeasurably when I listened to what my husband had been saying all along, and which I had dismissed as lazy cliche - viz: Young people (even old young people) are inherently and deeply self-absorbed. The degree of their self-absorption is quite remarkable. It doesn't mean that they don't love you (I know you're not exactly complaining of that, but just as shorthand) or need you to a degree that is impossible to overstate.
To come back to my first post on the thread - to some extent, one's children need to be able to not see you as not fully human - just as a thing, 'there', a sort of material force, or some other aspect of the laws of physics - in order to become themselves.
I think this can be incredibly painful to discover - I'm not saying 'ah, it's the wonder of growing a new human' or anything like that, far from it. But it is true. And the alternative - forcing them to see you as fully human with needs and desires and pains and a past - does not end well. They just experience it as a destabilisation at best, and at worst narcissism. I thought that if I explained everything mine would find it easier to understand me - but actually, the truth is they don't want or need to understand me in any meaningful sense, and they are damaged by being asked to do so, and/or by the information that they need in order to try.
Looking all this in the eye - a kind of parental nihilism - I think can be, if not liberating, then definitely a path to peace.
I hope some of this makes sense - it may not at all, because it's not universal. Or at least, plenty of parents either catastrophically underthink or healthily don't overthink these things, depending on your position on (psycho) analysis.
And also lots of children, either instinctively or because they are taught by example or worse, smoothe the way by being lovely and easy, such that no-one ever actually has to look into the abyss at the heart of the parent-child relationship. Lucky parents - neither I, nor my children, are natural smoothers-over of the tissue of half-truths that cover the abyss 😬😂