How can one know if God loves him? And is the decrease in commitment after marriage, despite supplicating for its increase, evidence of God's lack of love, or is it a trial, or a punishment?

As long as you feel your faith is deficient and you are pained by it, this is a sign of goodness in you. Seek out the causes of your waning faith and strive to remove them. The reason might be committing sins or being remiss in acts of obedience. Preoccupation with worldly matters, family, and wealth has a significant impact on faith.