Advice for trying to conceive.

So me and my fiance have been trying for a baby for about 3 and half months now with no luck. I started taking prenatals a month before we started hoping that would help. I even started taking ovulation test every month to try and pin point the best times to try nut nothing seems to be working for us. Ive been late a few time since starting and every time i take a test my heart breaks a little more with each result. His almost 4 year old son has even started to ask for a baby sister or brother and it hurts that i havent been able to give that to him yet. What should i do? Is there anything i should or shouldn't be doing to help us get pregnant? Any advice would be greatly appreciated.