Unable to suppress her anger any longer, she finally spoke up.
Tips
grammar
按捺 = to press down / hold under (by pressing). 不住 = cannot manage to (do the verb). Object is always an internal state: 喜悦, 愤怒, 好奇心, 激动, 冲动. Takes a direct object, no preposition needed.
memory
Imagine pressing a lid on a pot of boiling feelings - you can keep it down for a while, but at some point 按捺不住 - it pops off. The mental image makes the idiom easy to retrieve.
