It is he whom we proclaim, warning everyone and teaching everyone in all wisdom, so that we may present everyone mature in Christ. For this I toil and struggle with all the energy that he powerfully inspires within me. (NRSV)
If following Jesus obediently seems to be like hard work, that is because it definitely is hard work! We must never think that the biblical paragons of faith had it easy. Or that the great Christians we admire in more recent times are somehow immune from struggle.
Paul wants us to know his work of proclaiming, warning, and teaching is costly. Helping others to maturity in faith is not a piece of cake. Just think how hard it is for you and me ourselves to become mature!
He mentions toil and struggle. The original word he used is literally to agonize. Ministry to others is agonizing. Not because it is distasteful or despairing. Simply because human beings are all too human. Growth is difficult; throwing off old ways is challenging; surrendering to Jesus can seem scary.
But what saves Paul, and us, from dejection and utter fatigue is that as we work hard, God empowers us to do so. We do not do it all unaided. My powerlessness is embraced by his power. Following Jesus is hard work, and would be impossible work if it were not for his empowering presence. I work, God works.