I ate too much and it’s painful.
I'm agonizing over the fact that I'm not able to confess my love to Tofugu.
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This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of suffering? It's a description of that word, making it painful.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
Everything is really painful right now. Why? You have something heavy on top of you, crushing (くる) you.