I am so dumb and I'm pretty sure my iq is lower than average?

You gave no examples of why you think what you do. Based only on the wording of your question I would say you clearly have average intelligence. There have been many, many questions posted on Y/A that show a lot less intelligence than yours does.

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I suggest what you lack is self confidence not a lack of intelligence.

That’s how I feel sometimes because I failed in life several times but life is difficult. I can’t give up and you shouldn’t give up either. Be confident that your smart and the people who are making you feel dumb is dumb. 

Welcome to the club.  Exactly half the people in the U.S. are below average in IQ.

This self-awareness, if accurate, may indicate that you're not as dumb as you think. The really dumb ones are those who think they're geniuses and whose behavior indicates that they are anything but.

You are correct.  But fret not. SloJoe Biden has long been a running joke in Washington as the most likely to screw up anything, and look where he is today.  He has taken the Peter principal to its highest possible level. 

so what.  is that in the way of you being happy in life?

go test yourself. maybe you just have a learning disability. they come in many forms.

https://www.youtube.com/watch?v=-ptJ8UCbDGQ

Your EQ is more important that your IQ - and you can do something about it.