Learn English – “Struggle to” or “struggle with”

They have different meanings. A struggle for life is more specific; it means struggling to keep from dying. A person in danger of drowning will do his best to swim out of danger, and that action of swimming constitutes a struggle for life. So, your sentence has the meaning that if one is struggling to save one's life, what really matters is to keep from panicking.

The struggle of life is a more general reference to the fact (at least, what most of us believe to be a fact) that life is a struggle. So, it may also be true that what really matters in the struggle of life is to overcome one's fear, but it is referring to the more general struggle that is part of existence. Go after your dreams, so to speak.

Who is the "subjective" form (also called the nominative). Whom was originally the "objective" form (also called the accusative, but including what in other languages is called the dative as well). However, whom is increasingly replaced by who, especially in less-formal contexts.

The thing to understand is that while both the use of "whom" in the objective case and the avoidance of sentence-final prepositions are often seen in formal use, English isn't divided into two simple formal and informal registers.

Both "who[m]... with" and "with who" (which is unusual, but certainly permissible) could be considered as lowering the degree of formality , yet neither is as informal as it's possible to be. Both might be considered more formal than informal in some contexts.