There isn't an easy way of putting such a sensitive subject into words, but the time has come for you to take that step and move on. To move on and just break up with your friend. It isn't the easiest thing to do and I may be the wrong person to be even taking advice from with my track record on dealing with (not the prettiest.) There are a ton of different ways that this "break up" may happen, from distance, personality, or from having to choose to pick sides in a toxic group. These types of break ups are always the hardest to deal with than a romantic one though, and not many people understand that until they go through it.
When someone asks you about your break up with your significant other the most common answer would be "Oh it just didn't work out." When someone asks you about why you don't hang out with so-and-so anymore you can't just say "Oh it just didn't work out." It's just not that simple. Once you make a friend you don't really think about if you'll lose that friendship. Maybe you'll have a falling out at one point, but the friendship can always reignite. That's what friendship is. When a friendship gets to the point where it hurting you though, then it's time to step back and break up.
Dealing with losing a friendship is also way different than dealing with a romantic breakup. You're taught to get over a break up with your partner, but you're never really taught to get over a friendship with someone. Those heavy emotional feelings are just something that's completely different to handle. Society doesn't raise children to believe that they'll lose their friends.
The friendship doesn't have to be like a rollercoaster ride the whole time though. Deciding enough is enough is better than having the feeling of throwing up on a ride you can't get off. Maybe along the way of overcoming these hard feelings, you'll find new friends who can help you move on and make you feel like you, as a person and a friend, deserve to be treated better. You lose some, you win some. Having that mindset can make you move on with this tough and unexpected breakup.
