Sometimes friendships aren't meant to last a lifetime. That doesn't make them any less special, however holding to relationships that no longer lift you up or help you reach your goals can quickly turn toxic.
It's okay to walk away from those types of friendships in order to step into the next phase of your life. And you don't need to explain your reasons to anyone other than yourself. They may try to hold on to you because you do fill their cups, but they might not see how their negative energy is affecting you. There's no need to be rude when exiting these relationships, but be do be firm when telling them you are stepping away for a while.
Don't be sad for the end of the friendships, be happy for the memories and the fun times you had with those people. More importantly, be excited for the new people you can now add to your life who can teach you new things and help you along the path you have started down.
Some people want to stay where they are comfortable and that's okay... but that's not you, and it's a big step forward for you to realize who is holding you in place and who is helping you move forward.
You're doing the right thing, don't let anyone make you feel bad for looking out for yourself and your future plans.