over a year trying to conceive.

me and my partner have been trying to conceive for about 15 months and noooo luck I've done parental pills, switch my diet, started exercising and nothing yet. I've tracked my cycles for over a year and nothing its frustrating . this will be my fourth child but my partner's first child. So I would love to make him a daddy soon. but every month I get disappointed when I see a negative result.. What am I doing wrong.:(