1. You have several options here, but "no impossible" is not one of them. You could say, "not impossible", "nothing is impossible", or "no such thing as impossible". Why can't you say "no impossible"? In very general terms, "no" goes with a noun (for example: no salt, no hope, no experience) and "not" goes with an adjective (for example: not expensive, not bad, not difficult). The matter is not quite as simple as that, but within the context of your example, I think this explanation suffices.
2. Personally, I would have put "always" between "to" and "be" ("to always be"), as Elizabeth-Anne did, but that is our stylistic choice. I'm not sure that it is a grammatical requirement. I'm not an expert in grammar, however, so perhaps someone else will weigh in on this.
3. I would omit "a" in this case ("great power" or "great motivation", as Elizabeth-Anne suggested). Why? Generally speaking, uncountable nouns do not take an indefinite article.