Look at it this way:
You are being used and abused in this situation. - If someone you loved was in this situation and as unhappy as you, would you encourage them to stay, if so, why?
You are allowed to walk away from anything that you want without explanation. - You don't need to explain yourself because you already have and they didn't listen. They had a chance to listen and they didn't take it so that's their problem not yours.
You are allowed to walk away from a situation that puts your MH and physical health in danger. - You owe it to yourself to look after yourself because no one else will and those two have proved that they won't.
You are allowed to to be happy and to walk away from unhappiness. - No one has the right to demand you remain unhappy.
You owe nothing to those women at all, ever. You and DP are not married, you don't have kids. - There is literally nothing to keep you in this situation you have no responsibility here, you are allowed to leave as you please.
The only person keeping you there is you - and the biggest risks in life you ever take are the ones where you do nothing.
Think of it as a holiday if it helps. You have a flatshare so you won't be alone, you won't have cats and mums to consider, you won't have to pay off someone elses debts they ran up so you can spend it on something worthwhile like saving for a house, take up a hobby, or taking a really special women out on a fantastic date.
Give DP a chance to be happy with someone who loves living at home with mum and cats. Give yourself a chance to breathe and time to learn what you want from life and find the right partner for you.
After all is said and done what are they going to do if you just go in pack bags and leave? Put you in detention?
You could always leave a Dear John letter if you have to explain or print some of the stuff out that you have written here.