Hi @hellswelshy you are definitely not alone! If it's any comfort this is a very normal part of teenage development. As others have said, they need to prepare for separating from you. The teenage brain undergoes a lot of 'rewiring' with the result of them being very toddler-like. It is very common for teenagers to be selfish and self-absorbed. This does NOT mean you have done anything wrong in raising them or they will go on to become selfish adults, it really is a phase and they come through the other side.
I have 3 dds aged 20, 19 and 15. The oldest was a dream throughout her teen years, I was bracing myself for the worst but she really didn't change much (very unusual and I was lucky). However she did have some mental health problems around 17/18 which caused me a lot of worry.
The middle one had a difficult period from 13-15 ish but she was more unhappy than unpleasant. She is now 19 and at university and feels so much more secure than she did then. She remembers how it felt, hopeless and lost and like she was a horrible person so she created her own narrative. They really do test your unconditional love, they need to know however horrible they are, you still love them.
The youngest? Oh boy! It is currently hell-ish. She was diagnosed with autism at age 11 and that combined with teenage hormones is bloody awful. She is rude, unpleasant, swears, demands everything to be done right now and has little regard for anyone else. She has also created this narrative of being a horrible person and her self esteem drops. Sometimes when I say I love her, she says 'do you really?' (not in a sarcastic tone, she is genuinely surprised). They are in that in-between place of wanting to be independent but also wanting and needing their parents very much still.
I know how to handle her now - I don't initiate much conversation unless she wants it. If she wants a silent car journey so she can sit and think, so be it. She went through a phase a few months ago of not wanting hugs or any physical affection at all. But that's starting to come back now and she'll ask for hugs.
Up till recently I used to get very down about her behaviour. My mood would depend on how she was. But I read an article in The Times recently which prompted me to buy this book -
www.amazon.co.uk/gp/product/1472139011/ref=ppx_yo_dt_b_asin_title_o01_s00?psc=1&tag=mumsnetforu03-21&ie=UTF8
It's been an eye-opener and I've changed my attitude completely. You know that phrase 'you're only as happy as your unhappiest child'? Bollocks to that. I realised that dd's problems and issues (obviously manifested by her autism) aren't going to go away any time soon and there's no way I'm putting my life on hold till then. I've stopped obsessing about her behaviour and try and distance myself. I've told myself this is how things are now for HER, it WILL get better eventually but in the meantime I have my own life, my husband, other children and work and leisure pursuits. I think it makes it harder on you if you wait for change then you're disappointed that things are still shit. It's also not good for THEM to see their parents anxious and upset about their behaviour.
The books about teenagers mentioned above are good but I really like the book I linked to because it's more about how you react to their behaviour.
OP, as much as you can, leave them to it. Don't ask too many questions, if they want to talk they will initiate it. Be calm and consistent. Tell you you love them. I send my dd WhatsApp messages, cute GIFs etc which she likes. Just to connect to her when she won't talk to me.
Hang in there, as others have said, they do come through it eventually.