I started C25K and today I finished week 4.
I had some painful moments when I was working on week 2 & 3 and it is always going to be tougher - look back at week 1, if you had to repeat that week it would have been much easier!!
It's important to find the right shoes - usually after 700 km your shoes are worn out - so if you didn't change your shoes are old I suggest you change them.
Try breathing in through your nose and exhaling through your mouth, and don't give in to changing your breathing pattern - if you feel like you have to breath faster then slow down your pace.
Good Luck! Stay motivated