あまり

Expresses that a feeling or state is so intense that it leads to a result, often an involuntary one: “so … that …” or “out of extreme ….”

Somewhat literary, this pattern is common in writing and narration, especially with feelings such as joy or surprise. The following clause gives the consequence of the feeling’s intensity; the person affected need not be the speaker.

• あまりにN3「あまりに」is an adverb placed before a predicate to mean 'too...', with a neutral register. In 「あまりの＋noun」, when the noun expresses an emotion or feeling, it can be replaced with 「nounのあまり」—for example, 「あまりの人出に」cannot be replaced.
• あまりにもN3「あまりにも」is an adverb placed before adjectives and verbs, as in 「あまりにも嬉しくて泣いた」, with a neutral register. Besides introducing a result, it can also simply exclaim 'it's really too...', without necessarily being followed by a result; the two cannot be directly substituted—the sentence structure must be rewritten.
• すぎるN4「すぎる」attaches to the verb form with 「ます」removed, adjective stems, and the like, simply indicating that the appropriate limit is exceeded, often with a negative judgment; it does not necessarily lead to a result and is neutral in register. 「あまり」, by contrast, focuses on the consequences triggered by excessive degree and leans toward written style.
• ばかりにN2「ばかりに」does not emphasize excessive degree; rather, it states that a regrettable bad result came about simply because of some reason or quality (often a trivial one), and it cannot introduce a good result (except 「〜たいばかりに」). Nouns must take 「である／だった」.
• せいでN3「せいで」attributes a bad result to the preceding cause, with a tone of dissatisfaction; the cause is not limited to excessive degree, it works in both speech and writing, and い-adjectives attach directly. 「あまり」carries no blame, and its second clause can be a non-negative outcome such as crying tears of joy, so the two mostly cannot be interchanged.
• んばかりにN1「んばかりに」attaches to the negative stem of a verb and is an exaggerated metaphor meaning 'as if about to...', describing an action that does not actually happen; it is mostly used to depict others' demeanor. 「あまり」indicates a result triggered by excessive degree, so the two cannot be interchanged.

あまりに and あまりにも express excessive degree as adverbs, while すぎる marks excess; あまり here follows the expression of a feeling or state and introduces its consequence. ばかりに and せいで focus on a cause of a bad outcome, whereas んばかりに describes an appearance of being about to do something.