I consider only the present as existing.The past no longer exists, and the future does not yet exist. God is "I AM". He is primarily concerned with our present state of heart and mind. If we are presently loving and trusting Him we are in a healthy relationship with Him and have aeonous life. If we are not trusting and loving Him, we do not have aeonous life.
Hence, the only sin that has present reality is the sin one is doing in the present. So, when God speaks of forgiving sins, He is speaking of forgiving sins that are presently in our consciousness, either as a present memory or a present activity or a present desire. God does not impute to us any sin other than presently not trusting Him. That is the only sin that can separate us from God in the present.
So, God does not withhold forgiveness for past unbelief, since past sins, including past unbelief, no longer exist. Butwe need healing from any present guilt we feel over past unbelief. That guilt affects our relationship with God. That sense of guilt can hold us back from boldly coming to Him and from trusting ad loving Him in our present. The complete thorough-going forgiveness promised us through the cross, allows us to forgive ourselves for past sins, and to live in the present in a healthy loving and trusting relationship with God.
So when I say the only sin that God will not forgive is rejecting Jesus, I mean that the only sin that excludes us from knowing God experientially, that is, from aeonous life (John 17:3), is a present rejection of Jesus Christ as risen Lord and Saviour. I am not saying that our past unbelief will be held against us forever, and has the power to stop us having aeonous life in the present.
One of the effects of our mortality is for us to be distracted from the present, in which God dwells, to imaginations of the past and future which are not real but keep us from living in a trusting relationship the I AM. Aeonous life is me being an "I am" and knowing the "I AM" in the same present moment as the I AM inhabits.