How should I end it with her?

Hey All,

Long story short. I've been chatting with a girl for 18 months now and she keeps saying she loves me and wants to marry me etc...
Last week she did sexual stuff with a guy she met a couple hours before because he was hot.

I'm upset over this because I feel betrayed and she doesn't truly love me like she says she does.

So I need to end the friendship. I just don't know how.
Its online long distance so I could delete her but I feel bad just ignoring her. I don't want to upset her or surprise her as she doesn't knowkhow I feel about it.

Can anyone help?
How should I end it with her?
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