Remember that in this poem, the author was hanging in the balance between paganism (and the belief in Fate) and Christianity (where Fate doesn't exist, only the master plan of God).
The speaker of the poem is saying that Fate is stronger than any man's mind since he could have chosen the pleasurable and easy life on land, but went with his deepest desires instead. He returns to the sea, where he will most likely be hungry, cold, and exhausted much of the time, but happier than he would be on land since he loves the life on the sea. You see, it is his fate to be at sea...not to live a life of luxury and comfort on land.
Fate is simply whatever happens, and what happens is determined by many things including chance and mind. So fate is some thing passive that just reacts to other forces acting on it. While mind is one of the forces acting on the fate, IN this way I believe mind to be much stronger than fate.
Some people say that fate is something that is predetermined. Even if this view is correct that does not make fate any stronger than mind, because in that case the power of mind acting on the sequence of events is also predetermines. whoever makes the fate happen can do so only by making the mind act accordingly.
One practical suggestion, when making efforts it is best to consider personal efforts as the only factor a the factor that matters, and when enjoying the fruits of your labour it is best to consider everything you get as a gift of God.