Why is it so hard to conceive?!

I've been trying for two long years and nothing yet! I got pregnant once before about 6 years ago I miscarried.  I already talked to my OBGYN and he took a blood test and said I do ovulate. My BF has two kids from a previous relationship. I tried preseed my last cycle and that hasn't worked yet, but I'm not giving up on that yet. I also do ovulation kits. Idk what more to do I want to go to a fertility doctor but my medical don't cover that! It's just heart breaking every month I get a disappointment. I just really want a baby of my own. Any advice welcomed.