Seáno wrote earlier: Abair nach ndéanfaidh mé agus déanfaidh!
Now that you have provided the entire text in context, we can start over.
Abair ní feidir liom a dhéanamh é. Abair ní dhéanfadh mé iarracht é go fiú. Abair atá sé dodhéanta é. Abair atá an baol ro-ard é, an dúshlán chomh mór é, nó an éacht ro-crua é. Abair nach ndéanfaidh mé agus déanfaidh!
Literal translation: Say I am not able to do it. Say I would not even attempt it. Say that it is impossible (not doable). Say that the risk is too high, the challenge so big, or the feat too difficult. Tell me that I won't do it and I will do it!
Of course, there's probably some grammatical mistakes in my version. But a couple of notes:
If you are concerned about "dodhéanta" not looking right because it does not pair broad and slender vowels, remember it is a compound word, and compound words in Irish do not necessarily follow the broad/slender vowel rules.
I have included the "é" in my construct, because, as seáno has explained to me many times, it makes the reference to something specific, even though we might see it (and I used to see it) as redundant. It is in English, but in Irish it just means we are referring to a specific thing or action.
The Irish version repeats the verb phrase "do it" at the end because "will not" or "will" are components of the verb construct in Irish and do not exist independently the way we have them in English. You can think of the last "do it" as being parenthetical, and a good translator would possibly leave it out of the English translation from Irish.
If there are errors in my construct, I hope seáno will come to my rescue!
Dale D