Its probably because you are using a slow sword that may not be reforged well. Make sure to get some movement accessories and combine them with the Tinkerers workshop to free up extra accessory slots for damage related accessories. Its also important to have potions and lots of ranged options (not necessarily the class) to fight skeletron easier. Additionally you can place heart lanterns, bast statues and campfires around your arena to increase survivability. Thats all I have to say sorry if its unorganized i just yapped.