Viko: You are lost in the dreaded friend zone. You're her rock, her male shoulder to cry on, the one she goes to when she needs a male opinion, the one who boosts her self esteem, but you're not the one she wants to be romantic with. This is one of those relationships that every guy seems to find himself in at least once. Mine was in college and it was just like you were describing.
As for the flirting, girls to that all the time just to boost their own self-esteem. That's part of it. When we flirt, we want more. They do it just to show themselves they can. Don't read too much into that.
The girl I was talking about, I tried every way to get somewhere just like you're doing. Time went by, and we quit talking, and she happened to be a waitress at a place I took my date. At the end, she came up and shook my hand and pressed a slip of paper into my hand. Me being "Mr. Clueless" at the time, I opened it right in front of my date. Of course, you know it was her phone number.
We call this the "I don't want him but I can't stand to see him with anyone else" relationship.
Now here is where you have to man up. If you accept being relegated to the "friend zone," that's where she'll leave you. You, right now, need to move on. If she calls, tell her you have to go because you have plans. Let her know that you're not at her beck and call. That will make you more interesting and exciting.
One piece of advice I got when I was a teen was that a girl can't respect you if she feels she has you wrapped around her finger. You need to be a little contrary and difficult, just like you're doing right now. If she starts treating you like somebody, then you can reciprocate. Right now, you need to be aloof and send the message that you're not her servant boy.
I've done this several times, and it generally works. Throwing yourself at a girl never works.
Best wishes. If it's any comfort, we've all been through this. Just man up and she can't relegate you to the friend zone if you refuse to allow it.