كل شيء هالك إلا وجهه Everything will be destroyed except His Face. [Al-Qasas 28:88]
This verse describes that God alone will remain and nothing but "His face" will remain (وجهه). This clause is included to answer your question, that how can paradise be forever if only God is forever?
First of all, it should be made clear that heaven is eternal:
"But as for those who will prove fortunate, they shall be in Heaven; abiding therein so long as the heavens and the earth endure, excepting what thy Lord may will — a gift that shall not be cut off." (11:108)
"They will not taste death therein, other than the first death. And He will save them from the punishment of the blazing Fire," (44:56)
So how does the first verse not contradict this fact?
All the pious souls in heaven strive eternally to gain communion with God and get closer and closer to Him. They are in eternal obedience to God and never deviate from Him. Hence, they can be considered as manifestations of His Face (the greatest of which is Muhammad (sa)) and will remain forever enjoying His bounties because God lasts forever.
وجه may also mean attention, so only one who directs their attention to God and for whom likewise God directs His attention will remain. And everything else will cease to exist. We find an explanation along these lines in Bukhari Kitab ut Tafsir:
كُلُّ شَيْءٍ هَالِكٌ إِلاَّ وَجْهَهُ: إِلاَّ مُلْكَهُ، وَيُقَالُ إِلاَّ مَا أُرِيدَ بِهِ وَجْهُ اللَّهِ.
(Bukhari, Kitab ut Tafsir, under Surah Qasas)
إلا ما أريد به وجه الله – Except that which is done for His face/attention/pleasure (I'm not sure if this is a proper translation, I don't know Arabic too well, but I'm sure it means something along these lines).