Advice!!!!!

How do I tell my mom I’ve been dating someone for four months? She doesn’t approve of his age(he’s 18, I just turned 17) because he started college this year. But I’ve been talking to him for a really long time and I’m in love with him. He’s the best person I’ve ever met and I can’t imagine my life without him. We’ve been sneaking around for four months, but neither one of us want to do that anymore and I need to know some way to tell my mom but I don’t even know how to bring it up. I feel like a disappointment every time she looks at me and I don’t know how much more of thins I can take. I just want her to give him a shot. Any advice?