Need Help Getting Over Him

I recently broke up with someone I had been with for about 8 years. I guess you could say we were high school sweethearts. We both thought that we were gonna get married eventually in the future. We were very on and off though because we were kind of scared of how sure we were. We were scared that we were too sure and were ignoring other people we could have been with. At the time of our breakup we were technically "off", but we still acted very much like a couple and still loved each other a lot and still thought that we'd get married eventually. But then he developed feelings for another girl. So we had to stop doing the whole "off but acting like a couple thing" because he wanted to pursue these feelings. He said hasn't liked a girl this way since he liked me 8 years ago. Although he told me that he was as sure as I was that we'd get married, he has to pursue these feelings because what if we're wrong. And he's not the type of person to think something over and then just let it go, if that makes sense. He said that if this is a mistake and it's actually me the whole time, he wants to experience the mistake and not just hypothetically think about it. It's been hard for me to move on because one minute everything seemed so sure and then all of a sudden it was gone. We're still friends and we're going to try and stay best friends because he really was my best friend. We are trying to give each other space though, or at least he's giving me the space I need to move on. I asked him if he thought that this girl would be his new girlfriend and he said to me that honestly if things keep going the way that they are, most like in a couple months they will be together. My problem right now is letting go of this hope I have that he'll randomly change his mind and come knocking on my door saying "I've made a mistake, it was you the whole time". I mean that may happen in the movies, but it doesn't happen in real life. I'm not even bitter about the girl or him moving on. I just want to move on myself so I can continue being a good friend to him, but it's hard when I'm still stuck in this mentality that he was supposed to be my husband in the future. I'm not trying to find anyone new for myself because as much as I'd like to find someone new, I know that I'm not ready. But when the time does come for someone new to come into my life, I don't want to still be holding on to this bit of hope that him and this girl will breakup and that he'll come running back to me. I want to fully accept that us as a couple was just a season in my life that is now over and we're now starting a new season as friends. I've been considering getting therapy because this whole situation has left me depressed. Which probably isn't surprising since for 8 years I thought he was going to be it and all of a sudden the plan changed. I can't sleep, can't eat, and basically can't function because I'm having so much trouble moving on and realizing that it's over for good. But since I'm not in therapy yet, I was wondering if anyone had any tips or just similar experiences where they broke up with someone and were able to get over them and able to still be good friends.