I don't believe there is any such thing as "stupid". Define it without using it's opposite, "smart", as in "not smart". See, it's too vague and loaded with feeling words instead of helpful, descriptions. I would stay away from all words that are vague and pat like that and don't help you understand, they are just words that are used as threats or to hurt people and are, in a sense, like cuss words, which are usually used by people without enough "real" words to express themselves. Your father is verbally incompetent :-)

However, I would also stay away from alleged intelligence tests too to built your self-confidence and try to get out of your head some and all that thinking. No one can see what we think, only what we do. I'm not talking about you need to do things for your father better, to heck with him; how fast you find a pen for someone else when you don't want or need a pen, especially a particular pen, means nothing more than you say it means. You are the actor, if you want to find all pens quickly in a room, no matter who "hid" them, then you can decide you do that task well or poorly but you are your own actor and director in your life, not your father. He can only decide how well or poorly you do things the way he does or wants things and you don't want to be like him do you? He doesn't even take the tests you do; he doesn't find the pen does he? He would probably take longer

Pretend to agree with your father. Tell him, "Yes, sir, stupid is as stupid does" because you know you aren't the stupid one, he is (if there was a contest for "stupid" and/or anyone wanted to act stupid). If you did things really well that your father wanted, he'd have you do more things for him and you'd have no time to work on becoming yourself. You are not his brand of stupid but wear it like a cloak of protection from him. He can't see you and I believe is incapable of appreciating who you are. But just drop the worry about your intelligence and whether you can live your life the way you want to. That is your true task and I believe you are more than capable of filling it.

Your parents marriage is not your problem, it is your father and mother's. Your mother has her own true task and has to learn her own lessons in her life from it.

Useless, when applied to humans, is another word that is useless; you don't want to be used at all, you are a human, not an object.