I've BTDT! I just responded with my experience in another post above, but here goes again:
I had to go back to work when dd was 11 weeks old. We found a wonderful, loving woman to watch her. Our nanny was my age and she was married and wanted children of her own. In fact she had had a mc a year before. She was soooo loving and devoted to dd, that, at first, I was paranoid (until I got to know her better) that she would "steal" dd and run away with her. Crazy, I know, but these things happen. Once I realized that she wouldn't do this, I became very jealous. Nanny was super organized (more so than me) and took dd to parks, washed her after every poopy diaper (i didn't even do that), took long walks outside, played with her, held her until she fell asleep AND had time to clean up in the house. On top of that, I thought that dd looked more like her than me!! (DD favors dh and has very chubby cheeks, just like nanny)
I had serious separation anxiety because I had to go back to work. Add to that the fact that Nanny was so great and you had one seriously nutty mom on your hands!!
Fortunately, I was able to get a grip (after a few weeks of tormenting myself) and find my comfort level. I realized that nanny always deferred to me, which meant that she wasn't trying to replace me. I knew that she wasn't trying to "steal" dd from me (either literally or figuratively) She truly cared for dd, which meant that dd was content, happy, secure. She was careful, which meant that I didn't have to worry as much when I wasn't there. And, she was genuinely a nice and honest person, which meant that I was able to forge a good friendship with her. After a while, I felt more secure, dd was totally happy and it worked out well. I was really bummed (for myself and dd) when nanny told us she was pregnant and would be leaving us before her baby was born. Today, I still keep in touch with her and I have passed on a lot of dd's things for her ds. I feel like I really made a friend.
The moral of this story......we all feel threatened when our children seem to prefer somone over us......even though our rational mind tells us it isn't true at all!!
Those pesky female hormones always seem to mess things up!!