To help to cope you need the key to unlock the puzzle of what is happening, how your DS 'works' inside, and you also need access to the right support and expertise at school, later at work, everywhere in life. The dx provides that key. It is liberating, empowering. It also provides some legal protection.
I would totally second what was already said.
I think undiagnosed adults on the spectrum, who are successful, are coping, rightly consider their hidden ASD side as their individuality, their character, their strengths and a totally normal, essential part of human diversity. And all this is absolutely true. It is true after the diagnosis as well.
There are often question why there is the ASD 'epidemic'. The answer for me is that the modern life puts much higher pressure, continuous pressure on people, children, from early years and throughout various aspects of life to function exactly in the 'right way'. Everything is much faster, more pressured, measured more relentlessly and oppressively than in my childhood, that there is no respite, nowhere to hide. We are supposed to interact on ten channels, socialise, have superior emotional intelligence, social capital,... have likes on FB, followers, publish ourselves, be leaderful, jump through the hoops of school systems and work systems. We are continuously ranked. But all those things were designed, calibrated, modelled on the NT extraverted majority. So what was totally under radar for some of earlier generations surfaces now in relation to school, friendships, bullying, MH, work, relationships. You will find on MN threads in all topics with adults seeking advice in relation to ASD.
The thing is ,the diagnosis provides the key to understand and analyse why DC are the way they are, how it is right, but different, and how they need to act, what they need to do to cope. What the school needs to do.
It is much harder and potentially damaging without this key, because the 'problems' arise at the contact with the outside world, but there are no answers (none that apply).
I've seen with my DC that ASD related problems tend to pop up and snowball year on year as the pressure, the demands increase with age and the delay has personal cost. I was seeking early diagnosis for DD at 3, but was told to wait until she reaches 'the threshold'. She was diagnosed at 11, way too late as the lack of right support damaged her confidence, her MH, her learning. Despite understanding what was happening, I couldn't really help sufficiently without the dx and EHCP because the problems were at school, not at home, it wasn't entirely about what I do. Maybe that's just me and my DD, but I think it is about that external pressure that needs to be 'adjusted'.
I can see completely that it might feel revolting, it does for me, to see individual preferences, values, strengths being labelled as 'disability'. But this label exists because some of those qualities put many people at a disadvantage in society in its current shape. The legal definition of disability is achieving the same goals, doing things in a different way. DC need the freedom, and the support to function in the way that works for them.