Re: How to get over being left for someone else?
Whether he did or didn't is irrelevant, except of course, to you, because you have placed so much importance and have put your own emotional well-being on hold in your search for answers. And you've concluded that he didn't love you, and (in reading between the lines) you feel this is your fault, that there must be some lack in you that kept him from truly loving you.
And that's why I suggested therapy. You are assigning him WAY too much importance in your life. I know, you loved him (or still do?), but others have loved and lost and gone on to love again. Even when they didn't find out "why".
I don't think you need to "forgive" him, I think you need to give yourself a break. I can see you haven't reached "acceptance" in the grieving process. Plus you are hurting yourself, maybe as a form of punishment because this man (you've concluded) never loved you.
Give yourself a break, realize that no one and no relationship is perfect, and just because this man left you for someone else, it doesn't mean you deserved it or are unlovable. It happens every day to good people, but we can't let that define us.