TTC help?!

I've had 2 kids there ages are 4 and 5 but now me and my husband are trying for another (my last one lol)  it's been about a month and a half and I just feel like it's not working when I take a pregnancy test I get so excited only for it to come back negative.. We have sex every day but the last week or so it's every other day..and does this fertil aid actually work? Has anyone used it and any ideas on things that could work?!