Hi AGillian, and welcome to the forums. It can be tough to reach out and share what you're going through, so I'm glad you've been able to do that today.
While I do not have a whole lot of experience with this feeling myself, it is my understanding that it is something faced by a lot of people who have a mental illness. And even if it wasn't common, you are never being 'stupid' for the feelings you experience.
It seems like it must be tough to deal with this feeling in your life. While I understand it can be hard to challenge our thoughts, I want you to know that they are not always accurate or helpful. You do deserve to feel happiness, and I hope that is something that you are able to recognise.
It is my understanding that feelings like this are generally part of a larger issue affecting us. Have you seen someone for your mental health before? Getting help for your mental health seems like it could be beneficial in assisting you to address these feelings.