I think it's important to share some thoughts with you. It's better than stories. I'm sure the others did that already, and of course it makes you feel an affinity with others. I will say I've been there (releasing your first long term relationship). But there are some other important things. First.... no one said this yet...you're 18 you dated him at 15 1/2 (you said 2 1/2 years) and he was 19 going on 20? Or if you're already into the middle of your 18th year you were 16 dating a 20 year old. Either way... he got to you early if that's the case (and in some states I know that's legally wrong). This age range is definitely....though it seems small, it isn't...heck my fiance is about 5 yrs older than me (but then I'm in my late twenties so the gap is different)....what I mean is that a 15/16 year old is at a waaay different stage than a 20 yr old (no matter how mature). Why am I mentioning this? Because the way you're handling this is going to be harder because you started younger and were with someone who should not have dated you at the time. Your parents should NOT have allowed this, but that's hindsight.
I'm not saying it' s your fault I'd like to add. But my ex is that if you are 18 I'm sure you're not interested in what a 14 yr old is (they're just starting high school, you're in college), or when you were 16 you would Not date a 12 year old. It makes me wonder what was wrong with HIM to not realize that (you were a child). To me 16 is a milestone, 18 (as you're learning to be on your own and try to be an adult) and then 21 (which he was close to, ) is a milestone because about 20 you're deciding how to navigate the world: what you want to do: work? Continue to advance studies, etc. Whereas at 16 you're just learning about relationships (and you're really in developmental stages with them). And the mind can be easily manipulated and shaped at that age (especially if you're a woman). So you are thinking over this realtionship (it seems to me) and wondering what you did wrong and why you didn't let him go. Well you were immature, you're now developing (and maturing and will continue). So you really can't fault yourself for being 16 (I can fault him for being 20 he should know better by law he's an adult at that age and you were a child). Not that he's so mature, but you were in different stages. I think that's important to acknowledge so that you give yourself a break (and don't beat yourself up).
Now that that's out of the way,I'd say that to your first question: Yes. I have been blind to a relationship. I was your age (18) and guess what? It happens. Cause you're 18. I mean you're just learning how to navigate relationships and understanding who you are and what you want (and don't want). Now that I look back on it I take it as a learning milestone. At the time i spent countless nights analyzing and dissecting our relationship. Trying to figure WHY it hurts so much, why I even dated him, etc. But now I realize it's almost a right of passage. I don't think (very many) people get it right the first time out. It's only important what you do with the knowledge that you learned. In this case you recognize when you don't mesh with someone (for whatever reason) you don't have a relationship with them. You can't be right for everyone and there's nothing wrong with that. It took time for me too because I wanted to make it work. You can't MAKE something work when it's so wrong. It either does or doesn't (that's my belief).
THe answer to all those other questions is time. And it sounds like a cliche' but you move on. I think the best way is to rip that bandaid off. Meaning you don't linger, you don't have follow up conversations, you just move on (especially if you're sobbing every night). And it's okay to cry for a while, but I dont think it's anything to beat yourself up over. What I mean is mourn the loss of your first real relationship, but don't attack yourself while you do it. Sometimes things hurt, but one day you'll look back and not even remember that you were up nights sobbing (I was). You'll have moved on with your life. It's best to try to preoccupy yourself with friends, and fun movies (and believe me you'll probably cry through those anyways even if it isn't sad...or maybe it's just what I did. I remember watching stuff and something would remind me of him and I'd cry. Or singing a happy song and start crying. Eventually though I didn't think about him any more cause that's what happens.) but the more you focus on moving...I think you will eventually heal and move on. Laughter is the best way for me to get over something. Maybe it can help you. If you have good friends hang out with them, go to different activities, if you have a good family talk to them.
But really it's just time and that's the way it was for me.
In the words of a cheesy movie that I'll copy paste from script of rama as it sums up the hurt part best:
Kate:
Because I love him. And I'm afraid
that ...that I'll...
it'll hurt so much that I'll shrivel up
and never be able to love anyone again.
Luc:
You say that now but...
after a time, you would forget.
First, you would forget his chin
and then his nose
and after a while you would struggle
to remember the exact colour of his eyes.
And one day you wake up and he's gone.
His voice, his smell, his face.
He will have left you.
And then you can begin again.