There is no contradiction: verse 4:48 refers to the one who dies upon shirk without ever repenting from it — that person Allah will not forgive. But verse 39:53 ("do not despair of the mercy of Allah; indeed Allah forgives all sins") and 25:68–70 make clear that whoever repents from shirk or any other sin, even the gravest, is forgiven completely. This is why the Prophets called idol-worshippers to Islam and their entire past was wiped away upon accepting it. So the only truly unforgivable state is to leave this world still associating partners with Allah, unrepentant.
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.