Many famous mantras, tag-lines, titles of films and so forth come from poetry. Often not the lines that we'd expect.
A central theme that you seem to have for your character in their renewed life is the idea of perseverance or persistence. So I looked up "poems persistence" (without any quotes) on a well-known search engine.
The top result was a list of poems about perseverance from Discover Poetry. Here they are:
1.Persevere by Anonymous
2.Keep A-Pluggin' Away by Laurence Dunbar
3.The Hustling Pumpkin Vine by Ed. Blair
4.Perseverance by Alice Cary
5.Good Company by Anonymous
6.How Did You Die? by Edmund Vance Cooke
7.Keep a-Goin by Frank L. Stanton
8.Perseverance by Johann Wolfgang von Goethe
9.God Will Count Your Honest Try by William Henry Dawson
10.Endeavor by Anonymous
11.Try, Try Again by William E. Hickson
12. Boys Wanted by Anonymous
13. Effort by Edgar A. Guest
14. I Will Be Worthy of It by Ella Wheeler Wilcox
15. Clinching the Bolt by Edgar A. Guest
16. Earnestness by Ella Wheeler Wilcox
17. Resolve by Ella Wheeler Wilcox
18. His Other Chance by Edgar A. Guest
19. Do Your Best by Kate Louise Wheeler
20. Resolution by Ruby Archer
21. What the Birds Teach Us by J. R. Eastwood
22. Keep Going by Edgar A. Guest
23. See It Through by Edgar A. Guest
Without looking at any of the poems themselves yet, there already seem to be some possible candidates here. It might be that if the title is itself one of the lines from the poem, they could form part of a longer tag phrase. But I'd say at least these might be candidates for a character in one book or another:
- Persevere!
- Keep plugging away
- Keep a going
- God will count your honest try
- Try, try again
- I will be worthy of it
- Resolve!
- Do your best.
- See it through!
A lot of these are quite short, so without a following line from the poems themselves might not be quite what's needed. So: let's have a look inside some of these poems:
Perseverance, Anonymous
The fisher who draws in his net too soon,
Won't have any fish to sell;
The child who shuts up his book too soon,
Won't learn any lessons well.
If you would have your learning stay,
Be patient,—stick with it and hold fast:
The man who travels a mile each day,
May get round the world at last.
If you have a look in that poem, the first one on that page, I think there's four potential taglines there. Each rhyming couplet could be a contender. For example consider the last one:
The man who travels a mile each day,
May get round the world at last.
Or the last three lines together might be workable too:
Be patient,—stick with it and hold fast:
The man who travels a mile each day,
May get round the world at last.
And then there's all the other poems therein. Here's some potential tags from those poems:
Perseverance still is king, time its sure reward will bring: keep plugging away.
It ain't the fact you hurt that counts, but only how you take it.
Life's field will yield as we make it, a basket of thorns or of flowers
It's pluck that wins the day
Better far than brilliance is the effort that you make
Build on resolve, and not upon regret, the structure of thy future
So, how about the question, then?
Well, you can take the above as one kind of methodology that you could undertake to find a tag line or guiding quote. I think all of those poems are out of copyright. However, if you find a line in a poem that's not, all that';s required is that your character has read or heard a poem once. Remember that that was just the very first page that came up on the search engine!
Of course, in fact, you could write your own poem - either about the character, or by the character. Or by one of their friends or confidants. And then you can just steal a line from that. It might not be the line you foresaw when writing the poem. But that doesn't matter.
NB: this is not the only way, is maybe not the best way, might even be a poor way, but nonetheless is a relatively easy and enjoyable and effective way to find a mantra or guiding quote. Other places to look, of course, are famous religious texts and books of quotations. Good luck, and never give up!