In Wordsworth's poem "Michael," how does Wordsworth manage to make a hard life fulfilling?

Let me rephrase the issue at hand slightly: we'll look at how the speaker of the poem portrays a hard life as worthwhile. (You can make a good guess that the poet, Wordsworth, may hold the same views.)

So, the speaker sets the stage for this issue when he opens the poem like this:

"If from the public way you turn your steps
Up the tumultuous brook of Greenhead Ghyll,
You will suppose that with an upright path
Your feet must struggle; in such bold ascent
The pastoral mountains front you, face to face.
But, courage! for around that boisterous brook
The mountains have all opened out themselves
And made a hidden valley of their own."

He's saying that as you climb up a steep path, your feet have to struggle: in other words, a hard life requires strenuous effort. Mentioning "courage" and the beautiful scenery, though, the speaker hints that there's some value to be found in such a difficult journey. The rest of the poem should reveal what that value is.

As the story of Michael, the old shepherd, unfolds, we learn what a difficult life he has: full of labor, and of the loss of his beloved son, Luke. The things that make Michael's difficult life worthwhile and fulfilling, though, are central to the poem:

1. His deeply felt relationship with nature makes life fulfilling.

Michael's life unfolds as scenes on his beloved mountaintop, where he understands the wind and hears beautiful music in it:

"Hence had he learned the meaning of all winds,
Of blasts of every tone; and, oftentimes,
When others heeded not, he heard the South
Make subterraneous music, like the noise
Of bagpipers on distant Highland hills."

He probably feels alive and connected to the earth when he breathes in the air and walks with strength across the ground:

"Fields, where with cheerful spirits he had breathed
The common air; hills, which with vigorous step
He had so often climbed;"

Michael's deep affection for the pastoral setting in which he works is summed up by the speaker of the poem:

"A pleasurable feeling of blind love,
The pleasure which there is in life itself."

2. To a lesser extent, the pleasures to be taken in the simple act of eating also make life fulfilling.

"When day was gone,
And from their occupations out of doors
The Son and Father were come home, even then,
Their labour did not cease; unless when all
Turned to the cleanly supper-board, and there,
Each with a mess of pottage and skimmed milk,
Sat round the basket piled with oaten cakes,
And their plain home-made cheese."

These simple foods in abundance offer comfort and pleasure to Michael and his family, and are a source of joy in a life of difficult labor.

3. Lastly and most importantly, the joy of familial love makes life worthwhile.

Specifically, Michael's devotion to and profound love for his son fills his heart and mind:

"The Shepherd, if he loved himself, must needs
Have loved his Helpmate; but to Michael’s heart
This son of his old age was yet more dear—
Less from instinctive tenderness, the same 
Fond spirit that blindly works in the blood of all—
Than that a child, more than all other gifts
That earth can offer to declining man,
Brings hope with it, and forward-looking thoughts,"

The speaker of the poem, in the quote above, emphasizes how Michael thinks of his son as a dear gift that inspires in him both hope and a feeling of security about the future. As Luke grows up, he remains his father's "heart's joy" and "daily hope," and they even "play" together as companions. Even when Luke has to leave his family, both Michael and his wife Isabel are overjoyed to receive letters from him.

Despite the tragedy of losing Luke, and despite dying with his work unfinished, Michael finds joy, meaning, and fulfillment mainly in his relationship with his son and in his connection to nature.