First loves are things that stay deep within the skin. Sometimes, people just get under there and never really leave. That doesn't mean that it wasn't the right decision on either side to end the relationship.
Sometimes we romanticize past relationships and forget the bad, only remembering the good. Sometimes this becomes such an overwhelming feeling of having made a mistake, of feeling frustrated and upset. My first real love is still deep within my skin, despite having been in a loving relationship with another man for more than five years now. We've bought a house, got two dogs, recently got married, building a garden together and I'm really happy. But it doesn't meant that I'm going to be completely over my first real love. We're told that we must be completely over people at all times, but I don't think that's true, in the same way that when someone dies, we'll never really stop grieving, but rather, learn to cope with the grief.
A chapter of your life has ended, and it's really hard to get through that, to see the other end and to realise that there is more. So just take a deep breathe, let the feelings wash over you, and then let them dissipate away.