I had a cut on my lip and i was putting peroxide on it but i learned that kills new skin cells. i'm now using mederma for the scar but it doesn't look like it's going to heal the scar. any other suggestions or is the tissue too dead from the peroxide?

You are right: Full strength peroxide is toxic to new cell growth but generally won't destroy already living tissue. So you probably had some tissue that was already borderline in healing due to the injury. I don't usually recommend Mederma until the skin has healed or epithelialized.

Don't fuss too much!: Correct--full strength peroxide (3%) kills bacteria but is also harmful to new skin cells, and can actually slow or reduce healing (which is usually not a problem in a young healthy person, but can be an issue if older or diabetic). Mederma doesn't "heal" wounds--keeping the cut clean and moist with Bacitracin is probably the best. More antibiotics = more chance for allergic response. Let it heal!

Massage!: Don't beat yourself up too much about the hydrogen peroxide. It does slow healing of wounds, but lips heal so quickly, i doubt you did any harm. Scar massage takes time to work (2-3 months) and it cannot completely erase the scar. It will smooth and soften the scar and improve the color over time. Good luck!

Great blood supply: The good news is that area has fantastic blood supply and so healing will occur rapidly. Inflammation (red, hot, tender swelling) is the body increasing the blood flow to the injury to bring healing factors to the area and also the veins expand to carry bad stuff away. Dressings and ointments are of no value there. Avoid further trauma to the area and let nature take it's course.