I vote irrational (and I know that I don't get a vote!)
I understand that, a lot. I felt inadequate most of my life, and I didn't even have any guys wanting to be poly with me. It's usually an inside job. Came to its worst when I chose a verbally abusive partner, who simply 'proved' all that crap that I already believed about myself.
So, even if you think you're 'picking up on' something 'real' in the relationship, that still doesn't actually mean that you're actually inadequate. I feel quite certain in telling you there's nothing wrong with you. Unfortunately, I'm also quite certain that statement doesn't help. (sorry about that)
Anyway, there are a lot of people in the world who believe 'I'm not good enough.' And the answer is to unbelieve it. Who the hell ever let you believe you weren't? I, personally, think it is an unintended consequence of socializing children (from wild animals into civil human beings who can function in society with others). Parents must teach children how to get along and most parents have no training in how to do so without making children feel they aren't good enough.
(okay, I'm ranting now so I'll stop)
Very brave of you to look deep and find hard answers to hard questions.