I felt exactly this way, i almost left on 4 occasions, but finally after 4,.5 years i got there, and it does feel worth it in the end. Sometimes you need to take a step back and think about why you started in the first place. bascially with mine i spent 3 years being told i was not good enough by my sup, and it was a horrible time, however now i have a job in a similar field and am really good at it, which made me realise the challenge wasnt the phd it was the environment. i wasnt published, still am not, but i finished in the end. i always felt like i was not good enough but try not to compare yourself to others, what i realised in the end was some people are lucky and get results straight away, because they are following up something already know, i had to develop methods for a year which made me feel inadequate that i had no results, so dont compare yourself to others, and take something from the face that at each stage you have to pass review meetings and if your passing them then yo must be doing ok!