For my DC it meant that there was an explanation for how they reacted to things. DC is bright and for a lot of their childhood they were able to work out what they should be doing and almost “copy” that so they seemed like every other kid.
That worked really well for about 15 years but as teens grow the “rules” become more confusing - think about when you were that age and how different various groups of kids were at your school for example. They couldn’t always get the hang of “fitting in” and got quite isolated. Add in normal teenage insecurity etc, stress of exams to that mix and self-esteem and mental health can go through the floor.
Also as they become more independent the (in our case unconscious) support and accommodations that family and school bring fall away. School brings a huge amount of structure and that typically isn’t there at college, uni, work etc. That can create major stress for them. The extra effort involved in working out what to do can lead to anxiety, depression, self harm etc. The normal “bumps in the road” for growing adults can throw an autistic teen/young adult a lot more and their reactions to that can be intense.
For DC they’ve been able to understand now that there are certain things they need (eg some kind of structure for familiarity - that’s not necessarily what you may think of - they don’t have screaming meltdowns if plans change like you may be imagining autistic people do). They travel around the country and overseas independently regularly and whereas a few years ago a change in eg train times would have sent them into a panic and also made them feel enraged - they now know to take some breaths and logically assess the situation. They also know that if there is an “open ended” task, whether that’s an assignment, a deliverable at work, planning an activity or social event they need to put some structure that is meaningful to them around how to approach it - a deadline isn’t enough. These sound like kind of trivial examples but imagine most things you do creating stress because you find it hard to approach it - over time that becomes unbearable.
They also know that after a particularly draining set of social interactions they need to plan some downtime in and that that is ok. So for example when they were doing the “multiple gig economy jobs” they would offer up shifts that meant the one in hospitality was only done for a couple days and then they much more solitary one was done for a couple days. The autism diagnosis helped DC give themselves “permission” to do that.
It is hard to explain and also you have to bear in mind I don’t hear all of the things DC got from their follow up sessions as they were an adult when diagnosed. But having had a diagnosis of anxiety and depression as a teen they are now on much less medication for that than they were and in a much safer place.
Prior to their diagnosis it was obvious there was something wrong but it wasn’t being treated correctly (medication DC had wasn’t helping as they didn’t have the chemical imbalances that eg anxiety meds treat). An autism diagnosis followed up by a number of sessions to understand what that meant for them made a huge difference. I guess one analogy would be how therapy or CBT can help someone.
I get it can be a shock to have autism suggested but please don’t dismiss it out of hand. If it was just a label then that wouldn’t help but when a diagnosis comes with the right support for the individual it can be hugely positive.