I am a U.S. citizen and have been living overseas for already couple years and easily can pass the IRS physical presence test. However, what is not clear to me is that nevertheless I worked out of my apartment in a foreign country online, I did generate income mostly from US-based customers, which would report me (in fact a single person LLC company) as a US-entity in their filings. Do I have to pay taxes as usual in full or the fact I've been living in another country somehow mitigates my tax-burden?
US Citizens need to file taxes on all worldwide income, regardless of their residence. They may be able to obtain reduction on their net US taxes actually owed through effectively one of two methods:
(1) Calculation of their 'foreign tax credit' [essentially foreign taxes paid on the same income stream]; or
(2) the Foreign Earned Income Exclusion, which allows for ~90k of employment income earned outside the US by a non-US resident to not be included in your general income for US tax purposes.
However, for #1 above to apply, it will typically need to be income earned outside the US [if it is income earned in the US, the US will have the right to tax it first, and the other country may allow you to claim your US taxes as a foreign tax credit itself].
In either case, you still need to file the full tax return.
The LLC complicates things somewhat and I'm unable to comment on that point.
If you are uncomfortable spending 20+ hours on your taxes this year (making sure you research things directly from IRS sources and maintain a good file of records for future audit purposes), hire a good accountant. Good, in this case, unfortunately means expensive. I would expect at least $500+ to get someone who knows what they're doing. Might be more like $2,500+.