Why can't I pronounce the ŋ sound? (native English speaker)

I was wondering why it is that I'm unable to pronounce this sound. Apparently, the reason why I pronounce the words "seen" and "sing" the exact same way (as well as "long" and "lawn", "dean" and "ding," etc.) is because I have difficulty pronouncing the ŋ at the end of words that finish off with a g. Is it common for native English speakers - I'm from Michigan, by the way - to have this sort of dialect? And why is it that someone would have difficulty saying this sound? Is it fair to judge others who aren't able to pronounce an ŋ properly?

This picture shows the nasal cavity http://www.zompist.com/kitcons.gif

And here is a description. The description could be much better, but that's academic style. But picture and description indicate that the air passage through the mouth is closed. The air goes through the nasal cavity and the nose. We influence this by drawing in/back the tongue as I've just found out. http://www.pronuncian.com/Lessons/Default.aspx?Lesson=19 If you click "play" you hear the sound.

And the text also says that the vocal cords are vibrating, an important feature. The vocal cords are vibrating when you say a long vowel, e.g. aaaa.

Added: I had a closer look at the ng-problem, that is how we raise the back of the tongue. The description of pronunciation.com says "the back of the tongue lifts", but how? I doubt that anybody can lift the back of the tongue, the formulation is wrong. But you can bury the tip of your tongue as far down below the lower teeth as possible. This raises the back of the tongue automatically, if you want to or not. If you do this and try to say /n/ you produce the ng-sound.

I have seen this problem before in a friend of mine. She consistently pronounces all /ŋ/ as /n/ in word-final positions. She's from Colorado, so it doesn't appear to be region-specific.

I'm assuming you can pronounce /g/ just fine, since you didn't mention it. If you can pronounce /n/ and /g/ separately, then there's nothing stopping you from pronouncing /ŋ/ correctly (from a technical standpoint). Perhaps the rules your brain is using to pronounce "–ng" are just wrong and are converting it to be pronounced as "–n". It's hard to say with so small a sample size.

Can you pronounce /ŋ/ inside of words? i.e. does "ringer" sound as it ought to, or does it sound like "reener"? If the latter, then your issue goes a bit deeper is all.

I don't think it's terribly detrimental. Honestly, if you're old enough to be posting things here then you probably have made it far enough in life without it affecting you too adversely. With my friend, I really didn't notice until she did it back-to-back once (by saying something like "sing a song", which came out as "seen a sonn" and we all did a double-take), and that was probably a couple months into our friendship.

Like with most phonetics, it's something you can train yourself to do. You've just got to practice combing /n/ and /g/ to make /ŋ/. (I know that might be easier said than done, but it is possible!)

It is ordinary for word final velar nasal to become alveolar in an unstressed syllable, as in "licking". It happens in my speech, and I'm from Ohio originally. The vowel of the ending "-ing" is high and tense. I consider this to be standard English. There is a dialectal variant with lax "i" usually spelled with an apostrophe: "finger lickin'".

However, the change you report which even affects "ng" in stressed syllables, as in "sing", is not so usual. I hadn't noticed that pronunciation.

Often in languages, fewer kinds of consonants are permitted at the ends of words than in other positions. Many such cases are given by Trubetzkoy in Principles of Phonology. But I don't know why word final position favors such changes.