这是一个非常难得的机会,我一定要好好的珍惜它. 英语怎么说?

It is a very rare opportunity, I wil hold on to it carefully.(这是一个很难得的机会,我会好好抓住它的.) 前面两位说的都没有错， 不过英文没有那么。。。正规？的说法。 不知怎么解释， 不过一般如果你跟人家说英文是用上面的说法。 听起来会比较正常。（对了， 我的意思是楼上用的有点像中国的古文（比喻而已），你跟别人讲话是不会那样说话的）
by雁子~

有很多表达，我写几条
This is a golden chance, and I'll put up a good show
Chances like this are very rare.I will not let slip such a golden opportunity.
We don't often have an opportunity like this.I must seize the chance.
Here is a big chance.I must grasp this opportunity.

It's a once-in-a-lifetime chance. I'll definitely cherish it.

This is a chance once in a blue moon/It's a once in a lifetime chance.i really need cherish it.
once in a blue moon千载难逢

This is a very rare opportunity, I must cherish it well