Why its too hard to getting pregnant

I m 31 year old. I m trying since 4 year for conceiving I got pregnant before 1 nd half year but it was very weak pregnancy so I got miscarriage , after that I bleed since 20 days with pain in left side of my stomach. I consult with my doctor she checked and found a piece of fetus still in my uterus so she again did d&c procedure but its not working for me again . I bleed since 2 nd half month again with pelvic pain. I have arcuate uterus so she told that fetus tissue remain in ur uterus.after that m trying to conceive again but still I m trying