"Of course. They will be there for you no matter what you do and always support you."
I completely disagree. Friends will not be there no matter what, and no, they will not always support you. No matter what, you are not their first priority. First comes their own self, their family, possibly other friends, boyfriends/girlfriends, personal situations, school, work, other priorities, etc.
I'm in the same boat as you are at the moment. I saw that my latest "best" friend was using me for my money, trying to feed off of me, asking for things but never paying, etc. She only cared about herself - was sometimes there for me but her biggest part was listening, which let me tell you she did not even do because she was thinking about what she was going to wear or what guy she wanted to hook up with. Everything revolved around her. And even when I was in the hospital for 1 week, she did not even come to visit me or call. And because she was jealous of me, my intellect, my new job, my success in school and work, my reputation, my manners, etc. (what can i say, i am a good person with morals who shows respect to others, and I work hard in everything I do, unlike her) and my family and financial situation, she would try to find things about me and make me feel insecure..for instance, my weight or height (i'm short and a bit on the chunky side, but not fat at all by any means..I'm an 8 and wear s/medium shirts). So why be a friend with this type of person, or anyone you realize doesn't care?
A friendship goes both ways - it's a two way street and if you realize your friend is on a one way, move on because it's not worth it. Don't invest your time and feelings in these type of selfish, self-centered people.
The only people you can trust and know will be there for you are your family. And I don't disagree that friends are a good asset - of course they are - we need them for social growth and development and let's face it, someone to have with and talk to besides the family. And you should not feel bad because you can't "keep" a friend. If they are not there forever...they shouldn't be there. If they were friends, you would know and would not let them leave. Be brave.
Hope all is well. The time will come when you'll find someone who can be your friend for real, not like those we've previously encountered.