There was never a real line to be crossed. Nothing was said about what you could and couldn't do with other people. But that's the thing, you never did anything with anyone.
Accept him.
You went on dates, he took you to dinner, you spent nights talking about everything. He slowly learned who you were and that felt good.
Until you found out that you weren't the only one.
And sure, you weren't official, but you had offical feelings.
So you thought that he did to.
You were wrong.
And that sucks. You wasted time and effort and energy on someone who had no intention of being with you, at least not all the way.
It was complicated you say.
He's a really good guy, we were just on the wrong page.
No, he sucks for not saying anything, for not being man enough to admit that he had someone else, more than one person who he was seeing.
But because nothing was established you feel like you don't have any reason to be upset or angry.
You do.
It's not fun when your heart strings get pulled on.
And it's even worse when they break.
Stop making excuses for the asshole who played you. It's okay to admit that you were wrong. We all are, at some point or another.
But allow yourself to be upset.
Just because you weren't his "girlfriend" doesn't mean that you deserve to get lied to, disrespected and thrown away like you're nothing.
If he couldn't see what he did, he's a loser anyway. And trust me, sooner or later, he'll come crawling back, even if you don't notice.
So let him be for now. Let him play the field. Find someone who wants you, not just temporarily. Finds someone who wants to show you off, who introduces you to his friends, who really cares about you.
Until then, let the guy who you almost dated be just that. Nothing more.