There is nothing harder than having to let someone that you care about go. It sucks knowing that the situation could have been better but that person is not good for you. Everyone has to go through this, whether they want to or not.
The hardest part of the entire thing is actually letting of that person. Sometimes you don't know how to go about it and tell someone that you can't have them in your life anymore. Other times you have to give someone the cold shoulder and wait for them to notice. Even worse, sometimes you get into an argument and have a falling out. I have had all of these happen to me at least one time.
The worst feeling is having a falling out with someone. Most of the time, you want more than anything for the friendship or relationship to work out. Unfortunately, that does not always happen. Knowing you couldn't make the choice to let them go but they made the choice sucks. Not only did things with someone end, but they ended in an immature way.
Giving someone the 'cold shoulder' is extremely immature. I have to admit that I have done this before, but I would take it back if I could. Avoiding someone with no context and leaving them helpless usually occurs when someone wants to avoid conflict and confrontation. This usually results in an unhealthy relationship, making it that much more impossible to mend.
Telling someone that you do not want to be in a relationship or friendship with them anymore is extremely difficult. Having to tell someone that you feel they are toxic hurts not only them, but you as well. It's never fun having to do that. Sometimes the person will be defensive and tell you that they are not toxic, and to them they probably aren't. The most important thing is that you find that person toxic.
In the end, letting go of a toxic person is worth it.
I know that it might be hard to believe that you can get over that relationship or friendship with someone, but you can. Letting go of a toxic person in your life is extremely good for your mental and emotional health. You have to be able to overcome what toxic things someone has put you through, but it will make you stronger in the end. Knowing that you helped yourself isn't selfish, it is actually one of the most selfless things you can do. You better yourself for others in your life, and you are overall happier. I know firsthand what it feels like to go through this, and in the end, it is worth it.