How do you know when your friendship is over?

So basically I've known her for 10 years since I was in middle school and now we are in college. So far I haven't hanged out with her since 4th of July and when I did ask her to hang out we would make plans and on the day that we were suppose to hang out she would tell me that something came up. I saw her 2 days ago in school but she was with a guy. She called me over and told me to kill time with them until there class started but the entire time it was like I was invisible or even the third wheel. Then we ended up going to the library because it was hot and there was air conditioning in there but when we sat down they had fun by themselves and forgot about me. We spent an hour in there and not a single word to me. I just got sad that after 2 months we finally saw each other and I was just ignored. So I just got up and said I was leaving. She had a surprised face when I got up and left like she forgot I was even there. She told me what was wrong and I just said nothing and she said she was going to text me later when i left. I think she saw that I was sad when I left but she never texted me at all. As I left I could still hear them laughing. She has been the only friend that I've had for so long because I'm not good at making friends. Anyone I try to befriend ends up forgetting about me but lately its been feeling like our friendship is finally at its end after 10 years. I'm just feeling really sad right now. I just don't know what to do anytime I ask her to hang out she just tells me she is busy. Someone just give me some advice on what to do.