I'm sorry to hear that you are so down. Sounds to me like you maybe having "a mid life crisis." Do you know what got you in this state of mind? Anything can trigger it. I've been there myself and I've watched friends go though it too. I'm also 28.
It feels like nothing will every get you out of it, but you need to try. I still struggle with the day to day stuff myself. I don't want children but I see what other have (like a dog lol), their lives are in place where I'm still a drifter. Even though I have a boyfriend of 8 years.
I try to stop being jealous of what others have and try to set it more as goals. It can be really hard but you just have to drag yourself out of that "I'm worthless" state of mind and move forward. Things will be okay eventually even if it's not right now or today.
Hey
You feel that way because you think that way. Depression is a powerful enemy, it can suck the energy out of a person and leave a person feeling worthless and unmotivated. My advice to you my friend is to start doing the necessary measures to change your state. Consult a clinician and get diagnosis. Get prescription for necessary medications and undergo therapy. Start focusing on creating the proper mindset for a better life. Be proactive. Never give up and keep fighting. Don't let depression win.
Did you visit any GP, it looks like anxiety. You should talk to your family about the problem and go for proper treatment. You will be fine...
How often does this happen?