Guys approach me but it never goes any further because of my awkward nature?

I commute to uni and have met a lot of people that way. On my first day, I was approached by a guy who asked me about myself etc. It was friendly until he asked for my number. I got awkward and said "I don't know" and it got quiet and yeah. Awkward. Now, I know the problem here is me. I get very nervous and feel uncomfortable when approached. As a result, I'm rarely in a relationship. A few weeks later a different guy held a door open for me when going into a lecture hall. I said thank you, and he kept smiling. I felt more comfortable as I wasn't closed in on a train as before. He sat next to me, brought me an exercise sheet in the lecture etc. Just generally being gentlemanly guy lol. We see each other a lot and laugh every time. His friend came up to me and said that his friend said I'm "cool". We clicked instantly. I'm not like the other girls at uni in the sense of loudness or attention seeking. A lot of them throw themselves at guys and yearn to be noticed. I'm more quiet, read when waiting outside the lecture hall and try to keep myself to myself. I have quite a few good friends at uni but they rarely bother to come to lecture. Anyway, he brought out his phone one day and spoke about it (it was awkward) but didn't bring up anything about exchanging numbers. HOWEVER, I know that if the time comes I will close up and avoid him at all costs. This ALWAYS happens to me. If a guy shows interest, I feel incredibly awkward. Almost as though I'm unworthy of their attention. It's weird and I wanted to know if any other girl is this way.

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