Thank you for the question on canonicity, How can the Epistles be divinely inspired scripture if there are ones that have been lost, if they were, wouldn’t God have protected them (eg Laodiceans, previous letter to the Ephesians, the “Severe”)?
Here are some thoughts for one’s consideration:
A. The letters written by the Apostle Paul were addressing difficulties or giving encouragements to the churches/individuals.
B. It is likely that the Apostle Paul would have asked God for wisdom in responding to the needs of the early churches.
C. It is likely that the Apostle Paul wrote others letters that is not mentioned in his letters since he was considered their founding father.
D. In the preserved letters of the Apostle Paul, God in his divine providence allowed the churches to treasure/cherish the letters of Paul for his wisdom to the churches.
E. In the divine providence of God, God preserved the letters that would be most relevant through the ages. The others letters were more germane to their present situation of the first century, but may not be as relevant in the ages.
SUMMARY: God doesn’t make mistakes in preserving his words.
For further insight and discussion: