You will feel genuine grief for a good while, but it's OK to feel happy about your diagnosis too because while you can't change the past you can now make adjustments and accomodations for yourself for the future.
You'll go through cycles of thinking how did nobody know? Why did they treat me like this if they had any suspicions? Why has society expected me to behave in a neurotypical way? How will my family react, and will they accomodate me or will their perception of me change for the positive or the negative?
But now you know and you can put your own boundaries in place to make sure you are accommodated, you are regulated and your self worth isn't determined by neurotypical standards.
I am surprised that they used the term high functioning, because many professionals are stepping away from using functioning labels and instead focus on needs. Being high functioning is how others perceive you, especially when you're masking, but you can absolutely be struggling to function at all, whilst at the same time it can be true that you do have support needs but they're lower than someone who is non-verbal and will never live an independent lifestyle. Perhaps they just used it as a lay-term to explain how they've perceived you, but you should also remember they've only seen a few hours of you in a clinical setting. They've no idea what the rest of your life looks like.
I'm so happy for you that you've got some answers, and hope you've got a bit of breathing space now. It's been a big worry for you, but it's something you've known about yourself for a while and now you're validated.