i would lament!
sure, it'd be fine for a few years... then people would notice that you don't age... some government sponsored trenchcoats will abduct you and saw open your chest while you're still alive, trying to find your secret! when that doesn't work, some mad scientist will suggest liquification! maybe if they drink you, they will live forever too! maybe you reform from the liquid, but given the time course you might still be inside them... maybe you stay in the liquid state forever... assuming you reform they just keep experimenting on you! for years and painful years before you finally escape and in your RAGE burn the place to the ground, only to be sealed inside and crushed under tonnes of rock and metal for yet more ever painful years!
and when finally you are freed, and somehow find a way to live without anyone noticing, the sheer, crushing purposelessness of your forever unending life comes crashing down on you, you have all the time in the universe, nothing is beyond your reach, yet given everything, nothing is appealing, you learn and learn out of obligation, your curiosity smashed against the rocks by the perpetual tide of your life, until you learn everything, know everything, have experienced everything and nothing holds any meaning...
so you sit down calmly and wait for the world to end... as the earth is blown apart or engulfed in the sun, you burn for all eternity, or you spend eternity floating through a void that is constantly tearing you apart.. what happens to an immortal, reforming, undying body in the vacuum of space? does it adapt and become a cloud? or does the constant reforming keep your flesh in tact as the complete lack of pressure attempts to rend it from your bones? you end up floating through space disoriented as a cloud or in horrific pain, for billions and billions of years before to universe collapses... and who knows what the hell happens then
wow... that was a bit of a downer.
damn... immortality would suck!