Whereunto I also labour, striving according to his working, which worketh in me mightily.
Note 9 at Col 1:29: Serving Jesus Christ includes hard work. Paul used the Greek word "KOPIAO" to describe his "labour." KOPIAO actually means "to labour with wearisome effort" (Thayer's Greek-English Lexicon). In the Greek, this is in the present tense, which signifies a continuous and habitual action. Paul himself stated, "But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain; but I labored more abundantly than they all: yet not I, but the grace of God which was with me" (1Co 15:10, emphasis mine).
The word "striving" is also in the present tense in the Greek. This word carries the idea of competing in the gymnastic games and striving to obtain (Thayer's Greek-English Lexicon). Paul made it clear that Jesus Christ and His grace were the source of his strength and ministry. Grace understood correctly is not a passive theological concept but a life-giving power that manifests through mankind's weaknesses as they depend upon (exercise faith in) the living Christ (2Co 12:9).
Note 10 at Col 1:29: There is effort in the Christian life (see note 9 at this verse), but this verse makes it clear that our source of power is the Lord. The Amplified Bible puts it this way: "For this I labor [unto weariness], striving with all the superhuman energy which He so mightily enkindles and works within me."