We all are disappointed, let down, forgotten, and in other ways hurt by people we care about at some point in our relationships. It's human nature, unfortunately: No one is perfect in how they treat others, but trying our best not to hurt others is a good goal to keep in the front of your mind. "First do no harm" is often attributed to the Hippocratic oath, but that's not exactly true. Still, it's a good way to conduct oneself in most situations.
A common response to being hurt is to exclaim, "You hurt my feelings!" or "You make me so mad!" and other familiar lines we've said or heard, and then let oneself get worked up about the situation and the person or people involved, making us miserable. As much as it would be nice for someone else to be responsible for how we feel, feelings are really ours to own - which is really hard for many people. The truth is, our beliefs and our expectations are what's behind our feelings, which means no one made you feel anything. This is hard for some people to grasp and/or accept. They're not connecting that their "shoulds" (beliefs and expectations) for how other people act or treat them are at the core of how feelings arise in them.
The big thing that's not being understood is that by owning how we feel, we decide how we feel in any given moment which is very empowering, and keeps our stress lower, meaning we're happier in general. It doesn't mean we won't experience difficult feelings, it means we understand our feelings and deal well with them.
It takes work to deconstruct beliefs, understand where they come from, to work on responding differently to the stimulus in our lives, like someone forgetting our birthday, bullying us, or otherwise being insensitive, but it's possible, and it's worth it.
One simple way to start owning feelings is to stop using "You" statements and replace them with "I" statements. Starting a sentence with "You" is a sure way to make the other person defensive, and also usually just emboldens them to keep treating you in the way you dislike. Instead of saying, "You make me feel (bad, sad, mad etc.)", pause to think and assess, and then make a statement starting with "I", such as. "I don't appreciate the way you just talked to me. It didn't feel good, and I don't want you to do that again." That opens a door for real communication about the situation, vs. a "You" statement which will likely just start an argument or blame game that will result in further hurt and misunderstanding. Some might say this is unrealistic. It is actually very possible (again by taking responsibility), it just takes effort to slow oneself down a little and create a new way to manage one's feelings.
It takes time to learn this new way of responding and owning your feelings, but it's key to success in every type of relationship there is. And mostly, you will feel better about yourself and all areas of your life because you are honest with yourself and appropriately direct with others, you are living a life in line with your beliefs and values, and you are enjoying great relationships as a result. And, also flip this over and remember: You are not responsible for others' feelings, and don't accept someone trying to say you are.
Here are some detailed resources for working on owning your feelings:
Emotional Intelligence (find the section called "Emotional intelligence (EQ) skill 2: Emotional awareness")