Lost my friend...?

My friend and I have been best friends for about 10 years now. We're both seniors in high school now. Around grade 9, my friend just stopped talking to me. We went our separate ways for a little while, but when we started hanging out again, I found that they had new friends and a new best friend. It hurt, but I forgave and didn't ask what happened. My friend and their new best friend became really close for about a year (matching usernames, etc.) and no matter what happened it seemed like I was the third wheel and the afterthought. I stuck around anything, hoping I could maybe patch things up with my old best friend.
After a couple of months, my friend's best friend backstabbed them and ditched them. My friend was really upset for a long time, and often told me how that person was the only "true" friend they had. We patched things up after that person left and became best friends, although I still never asked why they dropped me in the first place.
At the beginning of this school year, they started to avoid me during school, although they would still text me after school. I discovered that they were now hanging out with their old best friend from before, the same one that backstabbed them. They avoided me, even getting up and moving away from me. My friend would say that they were too busy to text me, but would message that person instead. I eventually got up the courage to ask why they avoided me, and that's when my friend told me that they were now dating and that it wasn't fair if they couldn't have some time with just the two of them.
They continued to avoid up until about Christmas, with me trying to bargain and work out deals to try and be friends with my friend again. Around Christmas, I gave up and told them that it would probably be better if we just stopped talking altogether and go our separate ways. They said that I came before their date and that they couldn't handle losing their BFF. My friend talks to me more now, but still obviously sides with their date over everything and will flat out ignore/ditch me if their date is around. My friend and I were supposed to go to the same college, but they changed their plans to go to the same college as their date.
I don't want to lose my BFF, but being constantly ditched, ignored, and betrayed has taken its toll. We fight about it a lot now. I don't really even trust my friend anymore, but I don't want to lose them. What should I do? We graduate in a month and will then be in completely different states.