Every single sin has been paid for and forgiven, repentance is once, belief is once, the rest is discipleship and growing in the Lord and Saviour.
I see no scripture for your last statement, that believer loses forgiveness.
There are four types of soil in the parable of the sower and only one type endures bearing fruit to the end.
One of the types is not a believer at all.
Two of the types are believers in my opinion (the shallow ground and the thorny ground)...the plants grow but one dies quickly for lack of moisture and the other is choked out) for all practical purposes.
If one type the shallow soil, falls away, then that indicates that "believers can fall away"
And if another type is choked out by thorns then that indicates that believers can be overcome by the world.
It is only the fourth type of soil...an honest and good heart that bears fruit an hundredfold (Luke 8:15), that can be said to have eternal security as it is promised to us in such passages as John 5:24 and John 10:27-30.
Therefore, if two types of soil do not have eternal security, and they are believers, then a believer can lose forgiveness.