anybody having trouble get pregnant after miscarriage

so its been 2yrs since i had my M/C back in may of 2015...we've been ttc evey since with no luck and I've really been struggling with this you see it I was pregnant my sister found out she was pregnant but I lost my pregnancy and she went on to have my niece in December the same month I was supposed to have my twins..so everytime i see her its hard for me 😥😭and now she's pregnant again due in April am I'm just having a hard time being around them because i feel horrible and less then a women B/c i cant get pregnant again but then again i feel selfish because i have children and some women dont have any or have has so many losses that its too many to count and they just give up 😭😭i feel like i shouldnt dwell on the fact that I can't have another child and rather on the fact that I have two beautiful children and some women never get to experience that i just need advice any would be great thanks in advance