Is it normal to feel inadequate amongst friends?

I live with 3 girls, at uni. I'm close to two of them. One of the girls I'm close to is because we discuss our deepest problems freely. I tell her about my depression, in a way that I've never told anyone else. Therefore our friendship is valuable to me, because I've never felt so comfortable in being myself and letting it out. However, I feel, perhaps I'm wrong, I don't get alot of output. I feel the only time we can spend together, is our smoke breaks. And I don't even smoke that much, and haven't for very long.
I always feel like I'm intruding on her in her room, and outside of our smoke breaks, we don't spend time together. My other friend always looks like she wants to spend time with me. We've had ups and downs, but I've never felt insecure in that friendship.
I always make a huge effort. She never knocks on my door unless she needs to. Yet she always knocks on my other friend's door, the girl I hardly know. This girl has never made an effort to get to know me, because she has my best friend. My best friend fancied this girl at one point, kissed etc. This girl also has many problems, of which I don't know. I'm frightened because they are so close, my best friend might have said something about my own problems. Sometimes in the way she's acted, I feel she talks about me. I feel so inadequate next to this girl. I feel there's thin ice to walk on - my best friend is very easily irritated with things. I shouldn't feel like this, I know. I don't rely on her, because she told me, she wouldn't want me to be like that. Or is it her not wanting that? I know she has been good on times when I've needed someone, and I've gone to her room in tears. But I feel if I don't try, if I don't go to her, then...I don't know. Sorry this is so long.