They never achieve their stated dreams or goals. There is always something, more important to them, which holds them back. You could think of it as the difference between what they want - or say they want - and what is actually important to them.

The protagonist should always be slightly out of kilter with the sitcom world they inhabit. The adage of 'Ordinary People in Funny Situations or Funny People in Ordinary Situations' applies. However care needs to be taken so that the protagonist fits the situation well in at least one aspect, otherwise the audience may wonder why the protagonist stays in a situation where they clearly don't belong.

There is something they desire and in every episode they struggle towards it. They never succeed of course.

This would upset the equilibrium. It is only if you wish to create a new equilibrium that you should risk it.

More importantly should not realise why they keep failing. They should always be able to blame something else. If they realise what is wrong with them, they may be able to do something about it and that would never do.

They have always, in their view, made the correct choice. The fact that the right choice leads to failure is just one of those things.

Care should be taken that the Protagonists failure doesn't become obvious to the audience, otherwise they'll spend too much time wondering how the main character is going to muck it up this week. The failure should be inevitable but always surprising.

Often these are the main characters too and each would say that they are the protagonist and the other is the antagonist. They are both protagonists in their own stories and their own minds. Often an antagonist brings the protagonist down to earth and makes them see how useless they are.

You are the protagonist in you own life's sitcom. A basic guideline in almost all books and courses on writing is 'Write about what you know'. You are normally the person that you know best but you need to be honest.