Wow. In a few years she is going to look back at herself when she was 18 and go oh my god and hang her head in shame (hopefully). I don't think there is much you can do. You have told her until you are blue in the face and she is not going to listen to you, and you are getting hurt in the meantime.
I know it's hard to let her get on with it when you see the path in front of her and she doesn't, but some people cannot and will not be helped. They have to figure it out on their own.
Don't give up on her all together because hopefully one day she will come to your door having seen the mistake she's made and ask for your forgiveness.
Until then, don't let her have this effect on you. Don't call or text her, just make excuses when she wants to see you, because like you said you are probably just filling in the gaps for when he's not there.
One thing I will say, having been in a relationship which ended badly, yet we carried on 'seeing' each other, this can feel very degrading, yet you like the person so much you delude yourself into thinking this means you will get back together so you keep going back for more. At some point you realise it is just sex and it is a very sad feeling.
I know it's hard to let her get on with it when you see the path in front of her and she doesn't, but some people cannot and will not be helped. They have to figure it out on their own.
Don't give up on her all together because hopefully one day she will come to your door having seen the mistake she's made and ask for your forgiveness.
Until then, don't let her have this effect on you. Don't call or text her, just make excuses when she wants to see you, because like you said you are probably just filling in the gaps for when he's not there.
One thing I will say, having been in a relationship which ended badly, yet we carried on 'seeing' each other, this can feel very degrading, yet you like the person so much you delude yourself into thinking this means you will get back together so you keep going back for more. At some point you realise it is just sex and it is a very sad feeling.