actually agree very strongly with VM that the way the op asked the question was ridiculous. just because a person does an aveira even countless times does not mean that he isnt capable of solving anything. this highly incorrect attitude is part of what drives people off of the derech. you are writing off a significant portion of the population be very very careful.
i am sure the masseh with r’ elazar ben dordayah has been quoted on this subject a thousand times. a person can do tshuva on such a thing even later in life. one of the tricks of the yetzer harah is to trick a person in to thinking they are horrible and have no hope. if a person would be able to have the self-esteem to think they are someone worthy sometimes it is easier to withstand temptation as a person would say how can a person like me do something like that. have heard this shmuess many times and it is worth repeating for the ones who don’t know.