She’s worried he’s ‘out of her league’. What should she do?

“I have been dating this incredible guy for the last three months but I don’t think I can keep doing it. I feel like he is way out of my league and there’s no way he’ll want to stay with me forever. I’ve talked to him about it, but he only takes it as a compliment and tells me I have nothing to worry about, but I know I’m not wrong.”