I could feel my breathing speeding up为什么不能用breath要用breathing

breath——呼吸，气息，一口气；
breathing——呼吸（的动作）
现在一目了然了吧

选取牛津高阶英汉双解来作为解说的依据吧,比纯英文的词典更适合英语水平还不是特别高的中国人理解.
如果你用breath,那么在这个句子里面,my breath的搭配表明这个breath是名词.breath作为名词的解释如下：
(a) [U] (also infml 口语亦作 puff) air taken into and sent out of the lungs （呼吸的）空气:You can see people's breath on a cold day.冷天能看到人们呼出的空气.* His breath smelt of garlic.他呼出的气中有蒜味.(b) [C] single act of taking air into the lungs 吸气:take a deep breath,ie fill the lungs with air 深吸一口气.
而采用breathing的时候,breathing是breathe这个动词的现在分词或者动名词.
[I] take air into the lungs and send it out again 呼吸:People breathe more slowly when they are asleep.人睡觉时呼吸比较缓慢.* She's still breathing,ie still alive.她仍在呼吸（还活着）.* He was breathing hard/heavily after racing for the train.他跑着赶上火车以后,吃力地喘着气.
好了,不用我再多说了,我相信你已经能毫不费力的区分这两者了.