Have I been nursing for too long?

No. The aap recommends exclusive breastfeeding for all babies for 6 months, and that mom's breastfeed their babies until one year, or longer if both mom and baby desire. Breast milk is the best choice for every baby. If you feel that it's time to wean, or you need to wean and can't, please discuss this with your pediatrician.
Yes. For a new breastfeeding mother with nipple pain during nursing, it may be necessary at times to limit the amount of time that the baby stays at the breast. Usually, repositioning the baby will help as well. Many babies need "comfort suckling', which may mean they stay on the breast even after "being full". Please talk to your pediatrician, OB or lactation consultant if you have any persistent pain.