Then you should use texture mask on the default texture to completely cut out the separator with the sides and then cover the hole with the purple part
Lua Code:
local mask = self:CreateMaskTexture(nil, "BACKGROUND", nil, 0)
mask:SetTexture("Interface\\Addons\\tmask", "CLAMPTOWHITE", "CLAMPTOWHITE")
mask:SetAllPoints(self)
mainTexture:AddMaskTexture(mask)