Santification is a process.
First John is not a contrary teaching to this process.
1.) In the process even
faith is not stagnant but grows.
" ... because your faith GROWS exceedingly ..." (2 Thess. 1:3)
2.) The body
'grows" with the growth of God. The growing life of God is not stagnant throughout the church age.
" ... holding the Head, out from whom all the Body, ... GROWS with the growth of God." (See Col 2:19)
Related to the OP, this is why the Christian should seek the corporate experience of the Body based companionship, even on a two or three small group bases.
3.) No saints are instantaneously fully transformed. All need to be
"perfected" by the ministry of life in the normal church life.
" And He Himself gave ... shepherds and teachers for the PERFECTING of the saints unto the work of the ministry unto the building up of the body of Christ." (See Eph. 4:11,12)
Only those who think totally individualistically cannot see that spiritual growth and perfecting is related to the whole Body of Christ growing.
4.) Paul speaks of a point to which all growth will cause the Christ to arrive. It is not as if there is no need to
"arrive" apart from the process of sanctification.
"Until we all arrive at the oneness of the faith and of the full knowledge of the Son of God, at a full-grown man, at the measure of the stature of the fullness of Christ." (Eph. 4:13)
A collective, fully matured "full grown man" instantaneously at the measure of Christ's fullness does not result merely because numbers of Christians were evangelized.
5.) The need for GROWTH of that divine life is evident because
"God CAUSES to GROW" in the normal practical church life.
" I planted, Apollos watered, but God caused the GROWTH.
So then neither is he who plants anything nor he who waters, but God who causes the GROWTH" ( 1 Cor. 3:6,7)
Of course instantaneous sinless perfection upon being born again would not require God to cause to grow.
6.) Only those who think completely individualistically fail to see that individual sanctification is related to the whole building up of the living temple of God dwelling in His people corporately.
" For we are God's fellow workers, you are God's cultivated land [or farm] God's building." ( 1 Cor. 3:9)
So the growth of the divine life builds up the living
"building" of Christ's church. This is throughout every New Testament book a matter of process. There is no exception.
7.) Both Paul and John speak about levels of maturity. Though sometimes it is in a exhorting or even scolding tone it is still in love.
Paul speaks of the fleshy believers and carnal believers as opposed to spiritual.
Paul speaks of those needing milk still and not yet solid meat. This is an indication of levels of maturity.
John speaks in his epistle of
"young children, young men, and fathers".
It is ridiculous to imagine the fathers have not yet been born of God, the young men have not yet been born of God, and the young children have not yet been born of God. They have all been born of God.
8.) Since sanctification is a process Christians are exhorted to
"GROW UP INTO HIM in all things. " (Eph. 4:15)
9.) Milk of the word is not discouraged. Rather it is sought that it may CAUSE spiritual GROWTH. Of course newborn babes would not need GROWTH if upon being born they were instantaneously fully transformed.
"As newborn babes, long for the guileless milk of the word in order that by it you may GROW unto salvation." ( 2 Pet. 2:2)
10.) The process of sanctification requires the GROWTH in the grace and in the knowledge of Christ the Savior.
" But GROW in the grace and knowledge of our Lord and Savior Jesus Christ." (2 Pet. 3:18a)
11.) Paul expected the faith of the saints to
increase in the process of sanctification.
" ... as your faith is increasing ... " ( 2 Cor. 10:15)
It is not as if the Christians' faith is increased to the maximum immediately the moment of being born again.
12.) The normal Christian life is one of GROWING by the full knowledge of God.
"To walk worthily of the Lord to please Him in all things, bearing fruit in every good work and GROWING by the full knowledge of God." ( Col. 1:10)
Two points of this post are presented here.
Sanctification throughout the New Testament is a
PROCESS requiring GROWTH, INCREASE, DEVELOPMENT.
And the best environment for this growth is the corporate companionship with a view to God's desire to have a collective Body for Christ.
The OP about not being too hard on yourself means, seek companionship in love with other believers for more prevailing growth spiritually.