Simply being useful may not stop the feeling of being useless. In contrast, not contributing to society (leeching society's benefits) may create or amplify many negative feelings.
The essence of human productivity is taking one kind of good and turning it into another kind. In order to make some output, you need input. Take something or material, move it to a more useful place, or combine it with other materials to make something. Not all of us have access to the best inputs. There are no free inputs, but forward-thinking individuals have created things like the internet, which can be the next best thing to free inputs.
Consider the effects each of your actions will have on others.
Feelings are infectious. Smile and be polite to those you meet. If you have suppressed negative emotions, people around you will sense this and get "a bad vibe" , so find a friend to talk to.
Give more than you take. Selfless acts will create a mutual positive feeling.