I did some some research and you were right about "fantaisíocht" being nominative. "Fantaisíochta" is genitive, I think.

So, would it be more appropriate to use "fantaisíocht" or "fantaisíochta"?

The nominative.

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I recommend to learn Irish pronunciation on doegen.ie
Scottish Gaelic pronunciation on tobarandualchais.co.uk

Okay, thank you!

Here's the next piece:

I'm starting to realize.
I succumb to the loneliness.
The abyss of my agony.
My hopes have been set ablaze.
My fears have come true.
You have discarded me.

Thanks in advance.

I am a not very good translator but to start

Ritheann sé liom anois
Géillim don uaigneas
Aibhéis mo léin
Cuireadh mo dhóchas trí thine (literal) or "mheath mo dhóchas orm"
Fíoraíodh mo chuid eagla
Táim caithte agat (that last line, I think theres something better.... anyone?)

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I recommend to learn Irish pronunciation on doegen.ie
Scottish Gaelic pronunciation on tobarandualchais.co.uk

I like the use of "rith le" for the sense of that translation. I wouldn't have thought of it.
Also, "mheath mo dhóchas orm" is interesting, although I haven't looked into it fully yet. Maybe someone else can chime in on it.

The last line could be rendered so many ways. I've given some possible translations but I'm not sure which one would be best. I'm hoping we can get more people to see this and make suggestions.

You have discarded me. > "Chuir tú i leataobh mé." "Chaith tú uait mé." "Caithfidh tú mé a chur i leataobh." etc.

"Mheath a dhóchas air" is from teanglann.ie

Well, the best one we have is

Ritheann sé liom anois
Géillim don uaigneas
Aibhéis mo léin
Cuireadh mo dhóchas trí thine
Fíoraíodh mo chuid eagla
Chaith tú uait mé

I think. I can only confirm it is grammatically correct and comprehensible, but there are probably better translations for a couple of those lines.

_________________
I recommend to learn Irish pronunciation on doegen.ie
Scottish Gaelic pronunciation on tobarandualchais.co.uk