You need to think about what might happen if you break up. If it's mutual, and you decide to go your separate ways, fine. But it's often the case with such relationships that after a few years, he decides he wants to play the field a bit and leaves you to enjoy the single life again. You feel as if you've invested an awful lot in a relationship but all of a sudden (and often at a crucial time in your life) you're on your own again.
Worse, he feels guilty about wanting to leave and starts to pity you. If he stays with you because of guilt there's always the risk of infidelity: it happens more often than you'd think in such cases, because he stays but has to have his bit of freedom. It can lead to a lot of heartache for you.