First of all, "第二の人生" does not necessarily mean "life after retirement", although it's the most common usage of this phrase. "第二の人生" can be used at the wedding, the birth of the first child, finding a dream job, or whenever you want to say you are about to start a new life. Therefore "第一の人生" means the past period of your life, while "第三の人生" indicates that you have already experienced one life-change event, and expecting a new start in life. That's why no one says 第四の人生 or 第五 第六, because they are killing the audience's excitement and patience too. BTW, if you don't mind, "意分かっている" is not common in Japanese, you may say "だとわかりますが”,"を意味すると思いますが". : )