Let me start by saying mate - you opened yourself up to someone, showed your vulnerability, and allowed attachment to someone who you found yourself in. So when something like this does not work out, your mind sees it as a threat, and clogs up your thoughts with it.
Don't be frightened or confused by the thoughts, memories or dreams that may occur either. Very normal part of life. Little things will remind you of moments you shared with her or about her. A smell, something you ate, an activity you shared together. Disassociating can be difficult, but you can train your mind to think differently. Just do not let it fester and turn into something (like hate) because that can make it worse.
If you have any doubt about why its over, or about yourself or anything in that way shape or form - seek to find your own closure. I'm not saying go after this girl with an axe- you just have to find a way to close that chapter of your life and move on. Very important step. Understand the WHY so you can move past the NOW. The fact you're reaching out for help says it has deeply affected you for reasons beyond what you can comprehend into words, which can lead to self- doubt. If you know in your heart that you did your best, gave your all - then you can get past it on your own merits.
Some points I would share with you:
1) Avoid punishing yourself. Stop focusing on what YOU could have done better.
2) Reflect on how you are a different person from when you first started dating. Very critical - because sometimes, relationships sharpen us up for the next chapter of our lives, which turn out to be the greatest.
3) Time is your best friend, but also your worst enemy. Things will get better as time passes, but at the moment, it will feel like every day is an eternity. Hang tough
You're young AF. Don't look at your age and think to yourself you haven't got a chance to find happiness again. Take it from a bloke who was dating someone for 4.5 years, got engaged, only for it to go to shit not long after getting engaged. I have seen other people on here mention how their relationships fell apart for 1 reasons or another - and mate, it happens. It's a shit feeling, but light is always at the end of the tunnel. I was fortunate enough to meet someone less than 2 months after (sounds quick, but I knew that I gave my all, and deserved better for what I gave) - married 24 months later, and had a kid just 3 months ago. If a fugly mofo like me can do that, than everyone has a chance at happiness.