Miserable, Pathetic, Shameful, Deplorable, Wretched, Sad, Pitiable
い adjective
You've seen how the word 情け means pity, so adding ない to that turns it into an adjective meaning pitiable and miserable.
なさけない
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You learned this reading already so you should be able to read it!
今日もまたプレゼンでうまく話せなかったので、自分のことを情けないなと思いました。
I felt pathetic again today because I couldn't speak well during the presentation.
彼はその時、とても情けなさそうにしていました。
At that time, he looked very miserable.
情けないんですが、すぐに人を信じる性格なので、よくだまされるんです。
It's sad, but I have a very trusting nature, so I often get duped.