This is tough. There aren't any easy fixes. Emotions don't always make sense. Ignoring that doesn't help. Like you point out, a lot of people will say to just get over it, that she doesn't matter, she isn't important, that you'll "forget all about her," etc.
Those people are wrong.
In your entire life you will never ever have another first girlfriend. She was it. Yes, you can have other girlfriends. Maybe even better girlfriends. But she is and will always be your only first girlfriend.
My advice is that you accept that.
Yes, it hurts. And that's ok. Don't bottle it up. Allow yourself to feel what you feel. If nobody else understands, that's not your problem. Your problem is what's going on in your heart, and that's a problem you can't fight because it's part of you. It has to be accepted. You'll never get over this by denying what you feel and pretending it doesn't matter. It does matter. And that's ok.
Accept it.
Who she is and what she was to you will always be a part of you. You can continue to change and learn and grow and love again, but she will always have been an influence now along the path that leads to who you'll eventually become.
Accept it. And move on.
While she'll always be your first, that doesn't mean she'll be your last. You can have other firsts too. And those other firsts can grow to become more important than this first.
When you were a little kid, one day you took your first step, No other step you've taken in your entire life since, was that one. It was a super important step. Your first. Embrace that, accept it. But continue taking steps. It would be silly for a child to take his first step and then become so attached to it that he never walks again. It would be equally silly to be angry at that first step just because it's no longer with you.
So it is with love. Her place in your heart may never be replaced, but love is boundless.
You can love again.