I've read a lot of that stuff, and I'm a middle child too. From what I've seen, a lot of what is said about middle children is pretty much baloney. There may be some truth to some of it, but the same "truth" can also apply to people who aren't middle children at all.
Should you seek help? If you feel like you need help you should seek it; but I don't think you should specifically be thinking of "middle-child syndrome" if you decide to seek help.
I'm a combination of very independent and not necessarily needing/wanting anyone involved in some things; but also being very much someone who tries to be the "peace-keeper" (the books say middle children often are). For me, because I wasn't "the big cheese" at home, I think I found my "big cheese-ness" outside the house, making friends, being a little bit of leader among them, and having a great time. I get along with pretty much most people and enjoy socializing with a close friend or two. At the same time, I also enjoy my alone time.
I think if you're happy being alone you shouldn't let anyone make you think there's anything wrong with that. Then again, if you're not happy being alone and wish you could be different; maybe seeking help would be a good idea. Either way, go with how you feel within and what's in your heart - and forget about the books (in my opinion). (Part of being a middle child for me was knowing that if I needed someone to tell me I was "OK" or "a fine person", I had better be the one to judge myself, hold myself to my own standards, and then tell myself if/when I measured up. My mother was always kind of looking for non-existent problems because I was a middle child, and it irked the heck out of me that she didn't just see how OK I was. So, to me, being a middle child made me strong, independent, and able to define who/what I am for myself. )