Relationships are a large part of growing up for people. No matter how great those relationships can be in the beginning, sometimes its best to break up in the end. That's okay to do. You should not be unhappy in your life and if a relationship is causing that then it's okay to leave it. No matter if you're breaking up with your significant other or being the one broken up with, it is just as sad. Being sad is normal but so is being completely fine.
If you think about it, no matter the length of time you and your significant other are together there is some attachment to them there. After a breakup, friends are great people to turn to if you want to eat junk food, talk or even watch "How To Be Single" with you since you have no idea anymore how to actually talk to boys that are actually interested in you. Sometimes not doing anything is the best way for you to collect your thoughts and figure out for yourself what you want not only in the near future socially but in life in general.
Being sad does not have to be a public thing. Not everyone is the person who will openly cry about the situation and that is totally okay. Some would rather have a private moment to reflect on the memories or the problems in the relationship. Don't think you have to be open about why you broke up or feel bad that you feel fine about everything either.
It is completely normal to not be sad about a break up with your significant other too. Some people might think it is odd that you are two days out of a relationship and are ready to move on and have fun. If you want to go out on the town with the girls, then do it. Don't let the societal need for you to be sad keep you from living your life. You might feel guilty that you are out with friends having more fun than you've had in a while your significant other is at home sad and sitting in their room, but don't. People grieve differently and crazy as it seems, it's okay to know that your relationship ended for a reason and not feel the need to dwell on it.
Going through a break up is an adjustment for everyone both you and your former significant other know. The relationship is over, there are memories that you will have forever but there are friends who now don't know if it's okay to speak to you or hang out with you. Sometimes friends take it personally when you break up with a person and dislike you but that's fine. Let them feel the way they want because it no longer effects you.
The world is a big place. Break ups are a part of growing up. Your happiness is more important than what your ex's best friend is saying about you. It's okay to be sad that a once important person is no longer making memories with you but it's also okay to be happy and live your life to the fullest. Your life is only so long, make the most of it by being happy and having the time of your life.