I feel you. My 18yo stepson has just left for uni this morning, and me and DP were both ready for it.
I've lived with them for 4 years, and have done everything possible to help DP with him over that time (I WFH and he works full time so a lot of the household chores including looking after/doing things for DSS fell to me, I was happy to do it).
But. Since he was about 16, he changed completely. I think it was when he started sixth form and was given more freedom there. He would refuse to do anything round the house, wouldn't come out of his room apart from for meals, and just gamed all day into the early hours, until DP turned the internet off. He had to change the wifi password every time as he would just go and turn it on again after we were asleep (and then wake us up screaming at his game). He'd leave the kitchen in a mess if he made snacks, leave things all over the bathroom, his room was a disgrace..
It's been utterly exhausting.
It all came to a head when he turned 18, and he said he was an adult now and expected to be treated like one and make his own decisions.
Those decisions included not doing any housework or paying anything towards the house (like, he would expect to be reimbursed if he picked up a loaf of bread - because he'd eaten the last loaf), and instead festering in his room gaming all day and night while drinking on his own.
He was very VERY much ready for the student lifestyle, and I think that our relationship with him will be 100000000 x better when he's not living here - and fingers crossed living on his own will wake him up a bit before he comes back. I also hope he meets some new friends who aren't gamers, because he really really needs to find something else to do.