if you are confident, why are you heartbroken?a slut as described in your post is not worth your friendship;
a boy that can be seduced by her is not worth your love.
you love him, you dated him, but you just mourn on this forum about being afraid of him getting seduced by some sleep-around girl? why not enjoy life with him by telling him that you really like to become his galfren?
this kind of thing is not worth breaking your beloved heart.
the pre-assumption that the person actually has been and is her boyfriend now...
made her feel bad.