As the product of a single parent, I know how tough your situation is and my heart goes out to you.
It sounds like he has not learned / was not taught self regulation, and it sounds like this has impacted his anxiety, anger, motivation etc. In other words, he has a fragile sense of self and low confidence which manifests as poor mental health - but it does sound like it is mostly emotional abuse of you and his gf. Is he not used to consequences? It looks like he has never had to face up to anything - even his girlfriend bailed him out academically! What part time jobs has he had, and what chores did he used to do in the house before he went to uni? If things have only changed suddenly/seriously in the last couple of years, then that might suggest a mental health crisis. He still needs tough love to compel him to engage with help, because he cannot continue abusing his gf or you emotionally. If he has always been like this, then it might be his personality and you don’t have to like him - you can love him from a distance.
I agree with everyone else: you need to strip everything back. Give him 2 towels. Lock on your door. Don’t give him any money. Ever.
What do you think he is up to? Dealing? If so, this is the behaviour of someone who doesn’t think consequences apply to him.
What have you taught him about budgeting, bills and financial management? Does he understand the implications of blowing his student loan, or does he understand but blows it anyway knowing that he’ll be bailed? (Ie he has the skills and the understanding, but doesn’t bother because it’s fine as mum will bail him out) or does he need to sit down with a money management app and learn the basics?
Pick a calm time and discuss some basic, joint rules about how he wants to be treated at home and how you want to be treated. The key areas only: food, noise, cleanliness. Assert what you will accept but also make some reasonable allowances towards him. If he won’t, or he’s too unreasonable, then get through lockdown and kick him out. He’s not allowed to take advantage and people with mental health issues don’t get a free pass to abuse others. Unpleasant people can still have mental health problems, but it doesn’t mean you accept the unpleasantness. Likewise, those who are pleasant people can have poor mental health that sometimes (or even often) leads to poor behaviour, but they will show their nice sides too. For example, somebody with very high anxiety might be explosive due to their mental illness, but in between the explosive behaviour, they will show their underlying, real character and will be the warm, decent person that they are.
I am so sorry that you are in this position. However, he needs to grow up and learn to manage his mental health without taking it out on those who love him. I think you need more help than we can give, but at least you have some kind of sounding board here. It is so hard being on your own, and my heart goes out to you.