Why can't I make a best friend anymore?

So Im 18, college student and I do socialise with friends and family, when I was younger I had many best friends who I enjoyed being with and I cherished them, now Im 18 I find it hard to make a best friend even keep some friends like the other day I met up with a friend who is originally from Latvia she didn't have any friends in the UK and I was the only friend she had and she said I was her best friend, but now she has a new best friend and has forgotten about me, the point is people who I am friends with in the beginning seem to not want me as a best friend, but just a pal, do I need to look somewhere else for a best friend, what do you think?