I lurk on a here a bit, and it seems that oneitis effects many people in many different ways, but it is the main, overwhelming reason, for being beta.
To emphasise how beta some people are here is a story that I saw happen.
My friends and I are from Britain and we went to an American Spring break a few years back. On the second night my mate, let's call him Harry came back from the night out on his own. Got into a large hot tub where there were a few guys and a few chicks. Long story short persuades a girl to come back to his room. As he is getting out with the girl my friends and I are returning so he asks if we can chill for a bit around the pool whilst he bangs her. We're cool with it, but the girl has a guy friend that Harry was unaware of. This guy, let's name him Darren. Darren drove the girl for about 5h to get to spring break. They had been friends for many many years. And he clearly loved her.
So Harry and the girl go to the room and for a few hours we made some small talk and then we're like fuck it, we need to sleep. At this point Darren asks if he can join us in our room. We were like wtf, where are you staying? His plan had been to come to the party with her, and then leave with her and drive home. We didn't want to leave a bro out in the cold so we allowed him into the room very aware that the girl he loves is getting dicked by our mate. He knew it too, we suggested he sleep in his car but the girl had the keys. We offered to get the keys but whatever was going on in his head he wanted to be near her.
Go to the room and Harry is banging her in a bedroom with paper thin walls for all to hear(but not see), and we eventually get to sleep. Darren sleeps on the floor as we don't have another bed for him. I gave him my towel as a pillow.
So Darren is sleeping on the cold floor of some random people's hotel whilst listening to a girl he loves getting banged
Next day the girl gives car keys to Darren and says come back in a few days. She chilled with Harry (and us) for a few days and got banged repeatedly, she even made a few videos for Harry whilst they were fucking.
Few days later Darren returns picks her up and off they go. He is really angry with her, yet he still picks her up. Harry later tells us that Darren and the chick had a little falling out as friends because 'she ditched him'. (Harry is chatting to her via text for some reason)
Fast forward a year and Harry gets Snapchat from the chick of Darren and her being friends again. Just friends and he is so happy, yet we know that he loves her like crazy
Moral of the story
Don't be Darren - the girls a ho, she will happily be fucked by anyone but beta guys. Darren was stuck in some crazy world of oneitis and made himself beta. He would do anything for her. He drove 10h, twice for a shitty party. He slept on the floor whilst his oneitis was fucked next door. He was ecstatic at being friends with her again, but he will never fuck her even though he so badly wanted to. Don't be Darren
If you end up with a chick like this and she isn't giving you what you want, go for the next one. You have 3.5bn options. There is no 'one'. Don't do stupid shit like this and degrade yourself