Maybe the reason you don't do well is confidence. You try hard to convince people that you're so bad at debate that you've probably long convinced yourself. Now, INTPs are supposed to be Fe inferior, but I know I can understand peoples' feelings pretty well when I make the effort.
I'd suspect you just don't want to put the effort into debate, and it's not that you're bad at it. It sounds like a normal behavior for types that have Ti as a tertiary or inferior...they just don't like using that function, and it's not that they're necessarily bad at it. I'm the same way, I often just don't care about feelings and I often think they're unimportant to the current situation. I just feel more comfortable looking at the world as a big equation.
Forming a good argument is really not terribly difficult. It helps to have an intuition for it, sure, but understanding what it basically is, and then where people tend to go wrong in argumentation is all you need to really know.
An argument is just the logical conclusion given certain premises. If all the premises are taken as true, does the conclusion logically follow? That's the first question you have to ask. If the conclusion does not logically follow from the given premises, then the argument is logically flawed in some way. Normally people aren't just purely illogical, but instead when their argument fails it's because they commit an informal fallacy. Basically, their premises are just irrelevant to the conclusion...such as an ad Hominem (Well, you're not a biologist, so you don't know if evolution is true or not; therefore, evolution is wrong) and argumentum ad populum (Well, the majority of Americans are Christian, so God must exist).
After that it helps to question the premises...are they true? If one or more of the premises is questionable, then you point it out. If one of them is actually false or unknown, then the argument probably falls (unless the premise was irrelevant and the other premises alone prove the conclusion).