这句话意思是：当父母不容易,特别是当一个有特殊需要的孩子的父母常常负担着额外的压力.这里主要是泛指的一种额外的压力,并非特指（前文未提及）,而it和that都是特指,只有one可用于泛指.
以下是这几个词的辨析,希望有所帮助.
I. it／one ／that三者均可用作代词, 指代前面提到的名词.一般说来, it指代同名同物; one与that则指代同名异物.
I have lost my umbrella; I'm looking for it. （该句中it就是指前面的my umbrella）
I have lost my umbrella; I think I must buy one. （one在该句中表泛指, 因为my umbrella已经丢了）
The umbrella you bought is cheaper than that I bought. （替代词that在该句中特指“the umbrella I bought”, 以区别“the umbrella you bought”）
II. one与that虽可用来指代同名异物, 但one为泛指, 相当于a／an＋名词; that为特指, 相当于the ＋名词.所以one所指代的名词的修饰语一般为 a／an ／some ／any; that所指代的名词的修饰语往往是the ／this ／that.
A chair made of steel is stronger than one made of wood. （该句中one可以换成a chair）
The water in the cup is hotter than that in the pot. （该句中that可以换成 the water）
III. one只能代替可数名词单数, 代替可数名词复数时用ones; that既可以代替不可数名词也可以代替可数名词单数, 代替可数名词复数时用 those.
I like this pen more than that one. （one代替可数名词单数pen）
There were a few young people and some older ones in the house. （ones代替可数名词复数people）
Mary's handwriting is far better than that of Peter. （that代替不可数名词 handwriting）
These pictures are more beautiful than those. （those代替可数名词复数 pictures）
IV. one既可代替事物, 也可代替人, that只能代替事物而不能代替人.有时可以用the one或the ones代替that或those.
The one ／That on the table is mine. （该句中The one代替事物, 并且也可以用That）
He is the teacher, the one who is loved by the students. （该句中the one代替人, 不能用that）
He advised the farmers to choose the best seed-heads, the ones ／those that had the best color. （该句中the ones代替事物, 并且也可以用those）
V. one一般有前置修饰语, 有时也可有后置修饰语或不用修饰语.而 that不能有前置修饰语, 但可有后置修饰语.
Cook was a strict but good captain, one who took good care of his sailors.
The water in the well is cleaner than that in the river.
VI. it可以替代句中的不定式或从句等, 充当形式主语或形式宾语.one与that均无此用法.
It is known to everybody that the moon travels around the earth once every month.
I found it hard to get on with her.
VII. it与that均可以替代上文全句的内容或部分内容, 而one ／ones则不可以.
He has saved my life; I'll never forget it. （it代替第一分句）
Tom is painting his house. I am told he does it every four years. （it代替前面分句的部分内容painting his house）
Let's say we meet here at three o'clock. That ought to give you time to buy everything. （That代替前面表述的内容）