Go Away Self Esteem Monster

So, I'm sure I'm not the only one to ever feel this way... I might be the last of the fairly well-functioning 27-year-olds to, but whatever.... anyhow, sometimes I get these flashes of "I'm missing out" or "I was left out" or "Why aren't I the favorite friend?" and it drives me insane.  My rational mind says, "your friends/neighbors/pets/coworkers love you or, at the very least, respect you and laugh at your jokes."  But my irrational mind says "why aren't I good enough to be invited or why aren't I cool enough to go out and hang out and be interesting or why am I lame?"  I know, I mean I really do know, that I have great friends who love me and support me and that I am interesting, despite and actually because of my quirks.  But somehow, the feeling my Grandma passed on of "there's only so much love in the world and if you are giving it to someone else you aren't giving it to me" rears it's ugly head and I get so consumed with analyzing me (who I'm generally happy with, save about 20 pounds) that I lose site of my belief that there is an infinite amount of love in this world and I am lucky enough to be getting some of it from some wonderful people...

Does anyone else ever feel this way?  Or is it just me and my own issues?  How is it that I can wholly love myself for who I am and at the same time wholly doubt that I'm good enough for the people that love me?  What is that?