I take it that you aren't seeing anyone to get help yet. Do that. I'm no Dr. but your symptoms sound like depression to me. I've experienced many of the same feelings myself and it turns out that they were a combination of genetics and bad experiences. I found an excellent psychiatrist and obtained effective medication. I've never seen a therapist but I would if I could( not available here).
Obviously you aren't useless. You are father to a very young baby. You help mentally handicapped people. These are not small things.
What are you expecting from yourself? What do you want? First you have to find out the answer to these questions. You can't make a change until you know where you want to go.
Please see a Dr., I really think you can get some relief.