Trying to get pregnant!

For the past two years I have been trying to conceive after miscarriage in May of 2013. And so far nothing is working I have three children already and my husband and want to try for that girl. I'm 28 and I feel I'm not as fertile as before. We've had quiet a few pregnancys that didn't make it through out the years now I don't if I need a fertility doctor or just give up....