Feeling down

Alexandria
My fiancé and I have been trying officially TTC for 5 months now (we've never really used protection) just some background my mom had ovarian cancer when she was 17 but was able to have my and my sister before she had to have them removed. Since then I've got the 3 round of shots that helps to prevent me from getting it. Anyways we've been TTC on the days that best predict I will be pregnant and pretty much every other day and nothing has happened. I am 23 and he is 25 so I am having a hard time understanding why it's harder for me to get pregnant and it's hard not to get down when I'm not getting pregnant. Every time I see a negative on a test I always think something is wrong with me and makes me want to go to a doctor to just check and make sure. Everyone has always told me that it's really easy and to be carful. I've never tried an ovulation test but have though about it. I am seeking advice and words of encouragement.