To the person who feels like they've lost their best friend,
We've all been there, but let me tell you from personal experience, if you've lost them, you lucked out because they weren't really your best friend. If someone is truly your BEST friend, they will do anything in their power to make sure to always stay in contact, fix any argument, and always make sure you are taken care of. I don't know your exact circumstance and what exactly happened to you and your best friend, but I too have been in your position.
One thing I want to make a point of is, sometimes friends get distracted and busy and that's totally okay, you haven't lost them forever, they will be back and you will pick up right where you left. There have been times where I felt as if I had lost my best friend but once they got what they needed to deal with on their own, we picked up right where we left off. So, if they are acting distant, give them a little bit of time to figure out things going on in their life and see how you feel in a week or two. If you still feel as if they are being distant, talk to them about it. One of the things I've learned in the past 18 years is Communication is KEY! For any relationship, whether, that be a friendly or romantic one, it is SO important!
If you and your friend are in a fight and you feel as if it's never going to be fixed and the damage is there forever; just know that NO fight can't be solved. I know sometimes people can be stubborn, but at some point, someone needs to be the bigger person and make an effort to talk to the other. If you're the stubborn one in the friendship, try and push yourself outside of your comfort level and simply start by asking if you can talk to them. Sometimes the other person is waiting for you to make the move. Always try and talk out your fights whenever you can, if you don't talk and express your feelings with the other person, things will never get better. Trust me, from personal experience, I know how hard things can get between friends when you try and hide your feelings.
If you are in the situation where you feel like your best friend has replaced you, I know how much it hurts. I have felt the same way so many times, but it's best not to say anything at first and wait to see how things play out. It's okay for your best friend to have other friends, everyone needs to have different friends, that's just part of life. As long as they're still making time for you, it's completely fine! Sometimes friends get distant or sometimes they even need a break from each other altogether, but it doesn't always mean you're being replaced. If after some time you still feel as if you're getting replaced, reach out to your friend and ask to make plans with them. Sometimes people want you to be the one to make plans first and you might be surprised how well your next time you see each other goes! If you two are truly best friends you can never be replaced.
Friendships come and go, but if someone is truly your best friend they will always come back after being distant, you two will get over any fight, and you can never be replaced. Hang in there and try talking to them about your feelings. Things will get better for the two of you!