Hi Ben, firstly its great you feel confident enough to talk about this type of situation.
Secondly, I think you need to talk to your girlfriend about what she is doing, don't try to stop her because chances are she will just rebel and get worse; explain calmly one-on-one how it makes you feel and if she truly feels enough for you, she will take your opinion and stop.
If she wants to but cannot stop, seek professional help, and stick by her
Whatever you do, don't ignore or leave her at this moment because it's quite a crucial part of her life - try it once and stop, or carry on.
Good Luck
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