You don't tell him that you think you can "do better", that would hurt. But you CAN say that you have concerns "regarding the long-term feasibility of our relationship" if that's how you ultimately feel. People grow and they change as they do. Maybe you've grown in different directions. A lot of my posts are made with the assumption of you staying together, and I remember I said that would be the case, because that's where I am. But that doesn't mean that's the right answer for you. Is lacking marriage (voluntary) and a stable career a deal breaker for you?