못돼서 --> 못되다 I'm fairly certain in this context it has the meaning of being unfit. So like 'I'm just unfit to be with you'. 되다 has a lot of meanings, you should probably look it up in a dictionary to get the best one.
잘해주고 싶은데 -> 잘해주고 싶다 --> 잘해주다 (Be good to someone) +싶다 (want to)
'I want to be good to you.... but'