Love, at bottom, is desire, a strong yearning or longing for something, a passion for a thing strong enough to conform you in some way to it in your thinking and conduct. There is a human version of this love that is inward-focused, selfish, extended contingently to others and withdrawn easily. Godly love, agape love, in contrast, is a holy desire, a longing for what is perfect and divine that comes from God to us in the Person of the Holy Spirit (Romans 5:5; Galatians 5:22). Agape love prompts us to seek after God even when it is very costly to do so; it moves us to forsake anything that would interfere with a pursuit of God; godly love moves us always into joyful fellowship with God.
Having abandoned one's "first love" for God means, I think, to have allowed one's love for God to cool, to be superseded by Self-love and all the things that Self-love leads one to desire and pursue. This manifests in spiritual complacency and eventual moral compromise, as well as the development of an external, hypocritical "righteousness" that is motivated by fear, duty, obligation, and/or religious piety and pride.
An intense, over-riding, life-shaping desire for God is what a believer's "first love" is, arising in response to a knowledge and belief in the awesome love God has for His children (1 John 4:16-19). It is what the Psalmist was writing of when he wrote of his soul panting after God as the hart panting after the waterbrook (Psalm 42:1).