Oh tell me about it. One word: food.
Only saw the severity of the issue when Mumsnet posts talked about how they spend £50 a week on feeding a family of 4 or something.. it was a huge wake up call.
I find the key to losing all my hard earned money and come up empty-handed at the end of the month is always prioritising quality over cost (bread, eggs all the way to laundry detergent etc.) and zero junk allowed. That could see a packet of biscuits cost you £4.50 from a nice brand with good quality ingredients, instead of £1 for jammy dodgers at Iceland. And a packet of biscuits goes fast at my house..
Also convenience eating/drinking. It's scary how quickly the little things add up. I felt the same as you - how do I have no money left to buy myself nice things or go somewhere nice, having barely done anything but survive this last month? I sat down and went through all my outgoings and it just rinses out on convenience snacks and meals, coffees, grabbing a bottle of water when out and about.
Basically for some of us (I'm not the only one), once we think we can afford the finer stuff we go for all the organic, eco-friendly, delicious, or exotic stuff that we know if better for our own health and for the environment, because we feel we can. Then once we realise we ended up poorer than we've ever been, we've adjusted to the high life and it take a ton of adjusting to reign it back in!
I need to start with cutting out the snacks - refusing to snack will massively reduce grabbing stuff on the go and wasting my daily nutritional intake on expensive treats.