Yes, I know. I was the same which is why I’ve really wanted to keep it her choice. We really have to find the money for the swimming and the ones she misses keeps just costing us. I know we’d be paying it anyway but it’s a waste
You need to separate out the cost of the swimming lessons from the issue of whether or not she uses tampons. It's grossly unfair to link the two and christ knows what sort of hang ups it will be inducing in her. If you paid for a block of lessons and she got sick, would you tell her she was wasting the money? Otherwise effectively what you're saying is 'use tampons, or don't swim'.
This isn’t something I want to force her into if she doesn’t want them! She’s decided she wants to stop but it’s because of this and she’s upset about it but can’t bring herself to talk to me about it or try them.
What have you actually said to her then? Because you say you haven't forced her yet you've given her a 'choice' you know is untenable to her. Did you just say 'have you tried tampons, because it's easier to swim with them'. Or did you say 'have you tried tampons? You can swim with tampons in. Otherwise, we can't afford the lessons unless you go every week'. The second is very different from the first and does apply pressure. She's now feeling she has to give up her chosen sport entirely because once a month she doesn't want to swim. This is absolutely and completely the wrong message to send to any girl about her body, her choices, her periods and her sporting activities. Really, I cannot emphasise that enough.
Throughout my teens, there was nothing that would have induced me to talk to my mother about what I was, or wasn't, inserting into my vagina. She shouldn't need to say anything - it should be obvious why anyone would question using a tampon. You've said yourself you don't like the damn things. Leave her alone. And if you can afford the lessons pay for them, if you can't don't. But do not link the cost of them to her periods. It's cruel.