Heaven motivates Paul in ministry but ministry - and particularly the challenges and frustrations of serving in gospel work - motivates Paul to look forward to heaven.
The point is: Ministry is meant to be tough. We should be careful not to turn ministry into a career path, expecting things to get easier or more lucrative.
If anything, Paul reasons that God designed it to be this way - that the frustrations of this life would make us long for the permanence and joys of the life to come.