자신의 장점은 까맣게 잊고 (만다). = (They) end up forgetting about their own strong points completely. 만다 is the present tense form of 말다 (ㄹ is dropped when it combines with the -ㄴ/는 ending). It means "to intentionally avoid/stop doing something". It makes a strong negative by combining with -지. For example, 가지 마 (Don't go, asking the person not to go) is different from 가지 않아 (They or someone are not going - a descriptive statement). In "VERB-고 말다" form, it means doing something for the last time (i.e. it's done away with) either intentionally or otherwise, like taking the final action and forgetting about something bothersome (차를 팔아버리고 말았어), or ending up doing something by mistake or failure (시험에 떨어지고 말았다). It can sometime indicate a strong intention of a good kind, where 말다 connotes that it's so complete nothing further is needed. For example: 1. 맘이 안 맞아서 헤어지고 말았어 = We broke up for good because we couldn't get along. 2. 한번 쓰고 말 것도 아닌데 좋은 걸로 사자 = Let's get a good one - it's not like we'll use it only once. 3. 말도 안 되는 소릴 하길래 그냥 웃고 말았어 = They were talking nonsense, so I just laughed it away. 4. 나는 꼭 성공하고 말 거야 = I will make it no matter what (-고 말거야 expressing a strong intention)

No idea !!!