It's unhelpful perhaps to say this is the human condition, but your heartbreak goes back a few thousand years. It's there in stories, in the old songs that were passed round from mouth to mouth. It's at least half of the Blues.
Loving's like water, you turn it off and on
When I woke up this morning, found my good girl/man had gone
Love is teasing, love is pleasing
Love's a pleasure when first it is new
But as it grows older, love grows colder
And fades away like the morning dew.
And in these songs both men and women complain about the eternal infidelity of the other.
It must terrifying to find yourself in love with another woman when you could have sworn you truly loved someone - and you did. Love is not in the brain, it's not rational. It can strike suddenly.
But it does get better. I know everyone will tell you this, but that doesn't prove it's not true.
Time does heal; that's a universal experience. It leaves scars and you don't forget. Nor should you - that one is now part of you.