What can I take to make it easier to keep my eye open longer until it heals?

Artificial tears. I am assuming you have a corneal abrasion and the eye wants to reflexively close. Artificial tears are best. Topical anesthetics taken over a long period of time can cause problems like an anesthetic corneal ulceration. Sometimes, dilating the pupil can reduce the uncomfortable effect of bright lights.
Contact lens. If you have an abrasion, the best treatment in my opinion is a contact lens to act as a bandage/barrier along with a non steroidal anti-inflammatory eye drop (acular (ketorolac) or equivalent), maybe with an ice pack. If you are post surgical, ask you surgeon.