To be irritated by my boyfriend's daughter using my stuff?

No, quite normal

I don't share personal stuff either - definitely not without being asked

YANBU. I'd be annoyed too. I wouldn't want to share hairbrushes. His dd sounds very rude to take your stuff without asking. Best to take your stuff back home with you or have a little case that can be locked. You now know what to get her next Christmas 😊

But this teen isn’t there to ask, she’s also not related to OP, why doesn’t she have her own stuff? Why is it OPs responsibility? I think you are being a bit goady with your ‘my family is perfect and lovely’ crap. It’s not helpful and your situation is not the same as OPs, you are just looking to put her down

You’re right it would be nicer. But a simple “hey xx, would you mind bringing back my brush as I can’t find it”. Simple.

It’d be nicer if she atleast put OPs stuff back rather than putting it in her own drawers where OP can’t find it.

Absolutely. It isn't your house.

Nope. And yes I have teenagers and yes it happens. I ask them where it is and ask them to go and get it. No harm done.

I do think you’re being a bit miserable, yes. Mostly because of the “I’m not her mother!” - I was surprised to read later on you have daughters, are you like that with them as well?

If you don’t want her taking your things, put it all in a bag and get a little lock. That way she can’t really touch it.

Yeah but it’s her house not the girlfriends. So maybe in her house she sees them as her things. Maybe that’s how she was/is with her mum.

So you are honestly saying it wouldn’t annoy you at all if your teenage child was taking your things and then putting them in their own drawers rather than putting them back where they came from?

Oh okay then. Well for me I would have absolutely no problem at all with it and would speak to the potential SD about if she needs anything and ask her about what she is using and compare notes. What a nice and potential bonding experience - but then that’s looking at the positive rather than being upset about it and complaining.

But then my MIL was like that when asked to borrow a hairdryer. I underline the word asked.

Guess I’m just from a family where family (because let’s be honest it’s the daughters house not the girlfriends !) doesn’t mind these things. Not precious and don’t care - isn’t it nice to be nice to people you are living with at the time?

No they aren’t family. And it doesn’t bode well for the future if they become one either.

My god OP this would make my blood boil! YADNBU

Again, one person doing all the taking isn't sharing. This child is not really OPs family, from the sound of it.
It's also not very hygienic to share toiletries.
The hairdryer use is fine, as long as it's returned.

I'd do this. She might just need telling as kids often don't have great boundaries on their parent's belongings and she may be treating you the same. Can be simple as

"I'd rather you didn't use the bits I leave here please Lauren - I'm sure you'd rather I wasn't rifling through your stuff trying to find it either!"

You don't live there, you are only there once a month, why do you need to leave stuff there? Just take it home with you when you leave

No it isn’t. OP only stays there once a month and hasn’t seen this girl in ages so they’re not exactly family anyway

Guess we have different views of what family life is like then. I have no problem sharing my stuff with my family.

Buy duplicates for her
why should OP buy stuff for her bfs daughter she barely sees?
Helping herself to OPs belongings isn't acceptable, and everyone saying it's a free for all, this is how we have all
these entitled selfish teenager and young adults.
Tell her dad to speak to her and stop taking your stuff.

Ummmm, no it isn't.

It depends on your budget whether you'd view these things as inexpensive. Also, not sure how it's OPs responsibility.

Ummm yeah in a family it is.

Why you leave your stuff behind for once a month stay? Take it with you.

I share a hairbrush with DH but wouldn't want to share it with anyone else. I wouldn't share my deodorant with anyone though as it's a cream. It might be different if it was a spray but even so if she doesn't put it back I wouldn't be happy.

I think I'd just have a wash bag with essentials that I took there and then took home again to prevent her using my things.

These things are so inexpensive I would buy a shed load full and keep them out for her. I am surprised OP that you take this mean attitude when you have your own kids. In our house anyone can use anything unless specifically asked not to. My stuff is always disappearing but I just think we are a family life is too short.
Would you want your own kids to be thought of like this?

Honestly who are the parents here and what are the priorities?!
Newsflash: your BF is a sad cheapskate and a bad dad. (and you sound like a sad cheapskate and a bad mum!!)

Everybody has a budget but how does one treat the most important people? Surely not with pound store deo and shampoo?
I feel terrible for both your DCs who have to scrounge the girlfriend's supplies and aren't worthy of a hairdryer!

At your house, you should buy what your DCs need (nice stuff too, not just basics range from the £ store!) and leave it for them to use whenever they are with you. If nothing else, it lets them know they are welcome and loved rather than an inconvenience. And then you can keep your 'special' things in his/your room.

Likewise your DP should buy everything his DD needs for her to keep in her room just for her ... because he loves her and wants to see her and she's welcome to stay with him any time, right ...?

A box of supplies from the pound shop and a shitty little hairdryer is a mean insult, especially from you - just making her feel she isn't worthy of the best - and letting her know that she's not worthy of the nice things you have.

Background: my parents split up when I was 10. We went to my dad's every other weekend with a packed bag. My dad had lady friends to stay at his house on the weekends. None of them ever used my stuff and I never used theirs.
BUT, my DSis and I brought EVERYTHING every time we came to stay with him. We didn't have 'resident' toothbrushes, shampoo, etc. We brought a hairdryer every other freaking weekend because my dad never realised he could buy us one and we never dared to ask!