Hey Neo, take a deep breath and breathe.
Fear is to be expected! And all the panic and anxiety that comes with. It's easy to let our emotions run away, become something bigger than we think we can handle. I mean this IS big, but try and center yourself. You've got some changeling times ahead of you, so try to take care of yourself.
Do I have to tell my boyfriend?
Well no I don't suppose you have to. You don't have to do anything you don't want. But I will say (gently) that you cannot go on being a faithful girlfriend because you are certainly NOT that.
One of the BIGGEST downfalls of a WS is thinking they can control the outcome, or predict it for that matter, so we tailor ourselves and actions based on our perceived thoughts of a particular outcome we want or think we can achieve.
In your situation, you think he will be devastated and leave you. You've got the devastated part right. No way to skirt around that. But Neo, you don't know that he will leave you. WE all thought our BSs would leave. Some did, but hey, some didn't.
I was so sure on dday that that was it. He's done with with me, marriage over, game over. Well when that didn't prove to be true, I was faced with something else. Brutal honesty. That meant confessing to two more affairs. Holy shit Neo! I've never been so scared in my life face with that, oh I just knew if not before, now it will be over. I paced around the kitchen, heart beating, life flashing before my eyes, two phrases circulating around in my head; "fuck it" and "FL just do the right thing for once!". I finally blurted "there's more". I forced myself to go beyond the paralyzing fear. It was hard!
I'm always going to encourage truth over lies. So, I will encourage you to confess. It's not going to be easy, and it will get ugly and hard. But people do recover intact with their SO after infidelity. Please don't let your fears keep you from doing the right thing.