My daughter is always being left out

She's 9y and was my only child for a while.. I just had a baby. My youngest's dad has a 6y son that's here every other weekend(for the past year and a half). For the past few years my daughter has struggled with friendships. Making good friends and keeping friends. I don't really get it and don't know how to help her. I've struggled with making friends my whole life so I feel like it's probably something I passed on to her. I feel so bad and wish I could fix it. Ik growing up I was always left out. Only had a couple good friends and some of the people I thought were my friends joined in with people to pick on me or exclude me for no reason. Now I see and hear these same things happening to my daughter and it breaks my heart.. My daughter is sweet, strong willed, smart and pretty mature for her age. She definitely has some interests that other kids say are "weird" and she's a little more reserved/soft spoken at first.. she knows that everyone is different and liking different things is what makes us unique. Kids are just mean sometimes.. is this normal at her age? Any tips to help her?