Am I really supposed to love my boyfriend's children (daugthers 5 and 7) like my OWN?

He and I have two sons together...there is NO WAY I can love his or anyone elses children like my own. Especially his. When they come every other weekend I am NOT at peace. I'm uncomfortable in my home. My skin crawls. I'm jealous, I'm angry, I'm withdrawn. I'm on alert worrying about them interacting with my sons. Every other weekend is hell for me. We have them for a week toward the end of August,,, HELL.