trop difficile à tenir is tranlslated 'too difficult to hold'. "tenir" may also be understood as in 'tenir une promesse', to stand true to a promise or commitment.
trop difficile à tenir
Difficile
C'est trop difficile.
Difficile means 'difficult'
to hold
on est trop jeunes = we are too young
Too much of.
... qu'il est trop ... means ... that he is too much ... in French.
Il / elle parle trop means He / she speaks too much.
Tenir is a verb, not a noun in the French language. It means "to hold" or "to keep."
Tenir in French is "to take hold" in English.
too hard