It happened to myself as a 19-year-old with my first love. Twice she took my breath away, the first being when we met and the 2nd being when I learned she had been unfaithful. I was entirely over the moon for her. I trusted them completely so it was a total shock when I found out.
It was some time before I got over that experience and unfortunately, it played a significant role in the failure of other relationships that came afterward. I was untrusting, afraid of being hurt and so kept them at arm's length. A few years went past, perhaps 3 or 4 and I was able at that time to allow partners to get close and learned to trust again.
I've since forgiven my ex for their transgression all those years ago. She was 18 and even as adults people make poor decisions, so how could I still be angry over the decision of a teenager? Those are confusing years and at that age, we aren't always sure about what we want or how we feel.
My only advice to you Connor is simply this:
"O, beware, my lord, of jealousy;
It is the green-ey'd monster, which doth mock
The meat it feeds on. That cuckold lives in bliss,
Who, certain of his fate, loves not his wronger:
But O, what damnèd minutes tells he o'er
Who dotes, yet doubts, suspects, yet strongly loves!"
Othello Act 3, scene 3, 165–171