Do you get any large penalties using the greatsword one-handed?
If it's just a damage issue then the greatsword is still the better option. A +2 dagger stats 1d4+2 which is an average of 4.5, and the greatsword stats 2d6 which is an average of 7. The only thing that differs the two is a +1 to hit.
If AC is the issue I would just sell the dagger and go for better armor.