(Not sure if this is right section, move if needed)
Long time lurker, first time poster. I'll try to cut this short and get to the point. (I tend to go into detail)

I'm 18, just out of high school. I have no experience with girls other than friends what so ever(includes kissing). I'm currently pledging for a fraternity so meeting girls and getting their numbers isn't too difficult. I'm great with openers and continually engaging in conversation, it's when it's time to seal the deal I never go through with. Going to these parties, I meet at minimum an average of 7-8 girls, all who I get really close to. My conversations usually last a good 10 - 15 minutes before a mingle with someone else((usually talk about topics like pledging, greek life, school, major, drinking).

My self esteem isn't too high. My size is small, I have mild acne, and I'm a bit chubby. However, I am 6'1 tall and have a good sense of outerwear (((if that makes any difference))).

I feel like when I'm talking to these girls, I'm giving off too friendly of a vibe. Occasionally, our actives yell at us for NOT dancing with girls because it's something I'm uncomfortable with. I also think it's the fact that I haven't done anything familiar so I'm afraid how it'll play out the first time. I haven't taken to thought how pathetic I've really been until all my pledge brothers((even the most vile looking ones)) are hooking up.

tl;dr I'm pathetic at following through, how do I stop myself from backing out?

We've all been there.
Here are a few tips:

You just need to build your self-esteem. Nobody's perfect.
What you have to do is focus on minimizing your flaws while maximizing on your good traits.

For example:
I have a lot of natural confidence: smart, attractive, and not mention humble (haha). But I'm extremely introverted. So my entire M.O. involves showing off all of my good qualities without over-exerting myself socially. Hence I try to say well-informed things in a clear and coherent way.

_________________
The ultimate lesson of psychoanalysis is that human life is never "just life": we are possessed by the strange drive to enjoy life in excess, attached to a surplus which derails the ordinary run of things.
-Slavoj Zizek