The comparison of your children is unfair and will only breed contempt in the one being compared less favourably.
Your dcs are opposite sex and your ds and dd are at completely different life stages. Despite being 18, your dd is studying with people, who are 16 and 17. She will possibly be benchmarking herself against them. Perhaps she is also a lot less mature than your ds was at this age, but pushing against her isn’t going to make her grow up.
How much did you give to your ds when he was at home? Along with paying for his phone and bus pass, there will have been additional costs you paid. For starters, you will have fed him, perhaps paid for takeaways, took him out for meals etc. So in that regard, your dd is getting far less from you. Are you still getting child allowance for her?
At your dd’s stage of life, as long as I could afford it, I would expect to pay for all my child’s basic costs with a small allowance. I think £50 a month is a big ask for your dd to comfortably look after herself, especially as she lives elsewhere and undoubtedly has more on costs than she would living at home. Then there is the issues of costs your ds didn’t have. Even if your dd lived at home, £50 does go as for her, for starters, she will need sanitary wear, painkillers and bras. She probably also uses more personal hygiene items cleansers, face creams and makeup, all of which a male is far less likely to use.
Who is paying for her lunches when she’s at college? Is she expected to use the £50 for that?
Idk how much disposable income you have, but I got a similar amount to your dd and I’m early 50s. Clothes back then were about today’s prices, food and alcohol was obviously cheaper but the former not that much. I therefore used some of that money to socialise (ie booze and sometimes cigarettes) and buy clothes. I also worked at weekends and in the holidays to top up.
Tbh if I had an 18 year old feeling she needs to shout at me because she doesn’t have enough money for tampons, I’d be rather more introspective. I’m not saying you should allow yourself to be abused / shouted at. However, I think communication needs to be restored and a sit down talk about how much money she needs and what you need (ie calm communication) from her would be good. As the parent and adult, I’d be trying to get that to happen.
I would really try to dissociate the staying pretty much permanently with her bf from all of this. I know people are telling you leave her to it and she will come back. I’ve read enough posts on here to know that some women do not.