Forward I am heavy, but backwards I am not. What am I?

I hold memories of before, colors on a canvas kept safe behind glass and wood or steel or plastic.

You can make it pretty with paint you have bought In olden times two men have fought It's stone it's metal it's wood alike It's the only answer thats right

I arrive after bells and vanishing snow, Dressed in greens that the cold won’t know. They toast my coming with lights and cheer, Yet I belong to a quieter year. You’ll seek me in holly, hearth, and ring, But I’m not a holiday

Slayer of regrets, old and new, sought by many, found by few. What am I?