You have to focus on you and yourself and not worry about things on the outside. People will come and go and many will use you to get what they want and need and then disappear.
You are not a door mat so do not let them use you as one. You have feelings also and if they do not consider you and how you feel then they are not for you. A person that is for you will care for your well being and not just themselves.
Focus on your mental health and be sure you are okay. Take care of yourself and be sure that you are doing the things that you want to do, things that will make you happy and not wear yourself down trying to please everyone else. You cannot please everyone so please yourself.Heal yourself from everyone and things that broke you or you will end up breaking others around you and be sure that you love yourself, give yourself the time to heal and fix yourself before finding someone to love, this will help you not mess up but you can always heal with others if need be as long as you find someone who understands you and is willing to help you.
Healing yourself before jumping into another relationship will allow you to get rid of the excess baggage and will allow you to start over with the person but if you do not feel ready, do not jump and just wait until you are ready. Sometimes being alone is best.
It is not selfish to take care of yourself and say no to people, it is all about you and what you want not them.