Have you been droppin' your gs? Be honest. You probably do say "thinkin'" instead of "thinking" or "goin'" instead of "going" sometimes.
That's true for most speakers of English. In fact, both forms of words with "ng" endings have been around for hundreds of years.
Try making these sounds. First, run your tongue along the roof of your mouth and find the ridge behind your front teeth. That's called the alveolar ridge. If you put your tongue there and make a nasal sound, you get an "n" sound. This is the "n" that you hear at the end of words like "bin" and "sin."
Now, raise your tongue to the back of your mouth and do the same thing. This is the sound you hear at the end of a words like "sing" and "rung." They're spelled with a "g" at the end, but do you actually hear a "guh" sound? It's true that there are some varieties of English in which you'll hear "sing-guh," but most speakers say "sing."
What's interesting is that for most English speakers, the hard "g" sound will sometimes come back. For example, "long" has the same sound as "sing" at the end, but if we add "er" to make "longer," the hard "g" sound comes back. However, that's not the case when "sing" becomes "singer."