How can you forget something you've never done?

It's "haven't used it in a long time", look at the ずっと. "Never used it" would be 使ったことがない.
There's nothing special about this ~てない "form", it's just a contraction of ~ていない which you already know.
Also it's すっかり, not すかっり - there are
many words that follow this 〇っ〇り pattern.