At first it was great, now all we do is argue

well where to being well i been in a relationship for 8 months now and at first it was great we always did things like go for walks take the kids outside and never argued but know all we do is argue about everything and my daughter and her son are both 4 and feels like we have twins and its just becoming to much and very unhappy but i do love her and her son and she always on her phone when she gets home from work its just so very stressful its gotten so bad that  don't want to have s** anymore.

I realize I'm not perfect and not the nicest person to be around but she just nag me all the time and pushes me to my breaking point all the time well
any suggestion