Only then will I have a chance of achieving my dream

这句话的意思是，只有在那时，我才会有机会实现梦想。
only的倒装句。强调only后面的词或词组。
将only和only引导的词提前+助动词/be动词/情态动词+除了助动词/be动词/情态动词的原句。
原句是I will have a chance of achieving my dream only then.
例如 I read books only in the morning.变成Only in the morning do I read books.
He is a good boy only in his mind.变成only in his mind is he a good boy.

明白了吗？

if only是另外一种句式。表示：要是。。。该多好！后面要用虚拟语气。
If only there would be no school tomorrow!要是明天不上课该多好！

就是If only的倒装，If省略，Only提到句前，will 和 I 倒装

变回来就是

If only then, I will have a chance of achieving my dream.