You are misunderstanding what is being said.
Jesus says that He forgives all sins but one: Blasphemy. The easiest-to-understand definition of blasphemy is disbelief. Look no further than David the King, who was a murderer and an adulterer. . . whom God forgave. But David believed the Lord with all his heart, mind, and soul. David wasn't confused about the sins he committed. He never called wrong "right" or ran away from what he had done. One thing about David, he was very easily entreated back to repentance. And God forgave.
Compare that with. . . I don't know, fornicators, who even after being saved continue in their sin saying "it doesn't matter, it's not sin, God wants me to be happy. . . what difference does it make? Do those people with that heart attitude believe God with all their hearts, all their minds and all their souls?
Remember the parable of the seeds that fell on the stony ground? Remember Jesus said, a sower sows some seeds, and some falls on the shallow ground, it takes root for a moment and then withers and dies because it has no solid root system to support it. That is a class of people who believed in Jesus for a minute and then withered away. And then the seeds that fell in the thorny ground and the birds got to it? Same thing. They believed for a minute, but then the cares of the world choked them out. And then the rocky ground. Didn't even take root at all. But the seeds that fell on good ground were rooted, watered, nourished, grew.
Yes, Jesus says, In that day many will call to me saying Lord, Lord. . .but I will say to them, Depart from Me you who work iniquity; I never knew you. Do you think He's talking to the believers who were rooted in good, solid ground? Or "believers" who are Sunday believers, who are rooted in weak, shallow soil who never really "dug in" so to speak to our Lord?
We are saved by grace, Josh. We can't earn it. We can't work for it. It's totally by God's grace whom He will save, and whom He will not. Jesus says, Be assured of your salvation. So be assured. Dig in. Seek the Lord while He may be found.
Good luck to you in Jesus' name.