I think that Invictus is about how, no matter what life throws at you, you should always be yourself. It's about keeping your self-confidence, self-worth. It's about taking control, how it's your life, so you have to live it for you. There are mentions of life's struggles, and how it can be hard, harder than we ever expected. But, I think the main message is to always be you, and keep your own strength about you.
I really like this poem. I can relate to it very easily, and I don't struggle to understand the poet's meaning. My favourite line is "My head is bloody, but unbowed". That line really reached me. It tells me that no matter how many blows you take; how many scars you have (physical or emotional), you can still stand for yourself. You don't have to fall, break, simply because that's what the giver of those blows wants.
*Written for English class.