eternity

Dear Respected Scholars in Islam,Assalamu Alaikum, Until recently I have usually felt confident discussing Islamic beliefs with non-Muslims. However there is one issue that I am not able to explain to satisfy my own reason or heart. This is the belief of ?eternal? punishment for the disbelievers. It does not seem fair to punish a disbeliever for eternity, I can understand a very long period of time, but eternal punishment (even if it  was only one disbeliever) is a horror difficult to contemplate or rationalise against the All-Merciful (whose Mercy overcomes His Anger).Even, if a disbeliever underwent punishment in hell-fire even for a million years for every millisecond of his or her disbelief, then this would be understandable, but then this would be still a grain of sand compared to the never ending beach of eternal punishment. We know that even the lightest levels of Hell have unimaginable punishments. All of Islamic law is based upon proportionality and justice in this world, but this does not seem to apply to the crime of disbelief (as all other Sins can be forgiven after punishment). Although there are many descriptions in the Qur?an and Hadith regarding the nature of afterlife, I know it is dangerous to contemplate the issues regarding the unseen;  however the only way I can reconcile this issue with the beauty of Islam would be to say that disbelievers, though they will never taste the fruits of Paradise for their crime of Shirk, will undergo a unimaginably long punishment (within which they are neither dead or alive) and after which they will either cease to exist or perhaps a live never a ending existence that will neither be the severe torture of Hell nor the joys of Jannah. Please, I do not mean to offend with this question nor question Allah?s limitless mercy and forgiveness, I ask it in the utmost sincerity and zeal to seek the truth.JazakAllah,Allah Knows Best.Is it undoubtable according to Islamic text that people will suffer punishment for eternity, or are the words used in decribing the hereafter describing a very long period of time or even a indefinite period of time.