I'm the middle child, with an older sister and younger brother. It has never been a problem for me whatsoever. The only "issue" was that my mother (and others) were often keeping an eye out to see if I developed "problems" from being the "middle child".
I was at the top of my class as a young kid, kind of a leader-type among my girlfriends, got to spend time as "just girls" with my sister, and then got to play with my little brother when my sister was too old for playing. My mother used to mention how I was the "different one", but I think that was because second kids differentiate themselves for the first; and then youngest kids, in turn, differentiate themselves from the next one up (maybe - just my theory).
I don't really know what it is that paralyzes you, but I think only you can decide whether you will see yourself as a "loser". Yes, maybe older siblings feel more important; and maybe a lot of younger ones get to being "the big cheese" - but you aren't their view of you. Define yourself. It sounds to me like your family made too big of a deal of the middle-child thing.