HELP PLEASE!

Hey Ladies ! My period is 4 days late and I'm 15dpo. I took a pregnancy test at 13 & 14 dpo and got a negative test ( took them around 2:00pm). Usually my periods are never late, I dont know if I tested too soon? What do you girls think? Me and my s.o have been trying for 4 months and have had no luck, I get so sad seeing the negative test but since I'm so late on my period I dont want to give up hope. I was hoping someone could give me advice on what I should do? How many dpo did it take for you to get a positive result ? When should I try testing again?