Should I stop fostering?

Hi, I am a single mother of a 6 year old girl and have been licensed to foster and adopt for almost a year. I had two respite placements (5yr girl, 2 yr girl) a few months after getting my license. Last month I had a 7 yr old girl for only two weeks before she was moved to a relative's home. Now I have an 8 yr old girl (less than a week so far), and I am not sure how long the placement will last. My problem is that I wonder if I just don't have the right temperment to foster. I originally decided to foster for two main reasons. 1) to help a hurting child and 2) to possibly grow my family through adoption. I have always thought of myself as a compassionate and caring person. I thought I would be a nurturing and empathetic foster mom. BUT I have found that I am less patient and tolerant of my foster children's behaviors/habits than I am with my own daughter's. My first respite placement was 5 days and the girl kept saying "right?" after every thing she said the entire time she was here. It was so aggravating, and I hate to admit it, but I was so glad when it was bedtime each night. The 2 week placement also had irritating habits that got under my skin. I feel HORRIBLE for feeling this way. I know it is petty. Both of these girls were well behaved and got along well with my daughter, but I just didn't "click" with them. Is this normal or am I just not able to parent children other than my own? I have not felt affectionate towards the children (except for the 2 year old), but I am very affectionate with my daughter. I had to force myself to be affectionate with the foster children (one had scabies so that was a concern also). I know I sound so heartless, and I am disapointed with myself for not feeling anything for the girls. I did feel empathy for them, took good care of them physically, and even cried when I found out the 2 week placement was leaving to go to a relative. I don't want to continue to foster if I am unable to meet the children's emotional needs. Do you "feel something" for each child placed with you? I do care about the girls and want what is best for them, but I don't get beyond feeling that I am just babysitting or having my daughter's friends over. Please share your experiences with me so that I will know if I am normal or if I should quit fostering. Thank you!