까먹다 is the casual, slightly self-mocking word for forgetting, blunter than 잊어버리다. The conditional names the moment it slips away: 돌아서면 까먹고, "I forget the second I turn around"; 좀 지나고 나면 까먹어요; 다음날 되면 까먹거든요; 약간 흥분하면 까먹어 가지고. It belongs to speech — you would not write it in anything formal, and every line here is conversation.