Boku no ikirushirushi

[Cereth]

A friend sent me this lyrics, I think I understand it, but the last part is not totally clear for me Boku no ikirushirushi....I think that is a very important pharse, I appreciate your help.

Here is the context:

じぶんのおおきなゆめを、おうことが
いままでのぼくの、しごとだったけど。
きみを、しあわせにする、それこそが
これからの、ぼくの、いきるしるし


Thanks in advance!

 

[Flaminius]

Sorekoso-ga korekara-no boku-no ikiru shirushi.

Ikiru shirushi is "the sign that I live for/by/with" or raison d'être. Some possibly unnecessary comments for understanding the phrase;
koso: emphasis particle. Here it would be, "That [making the lover happy] and only that is the purpose of my life."
korekara-no: korekara, as you probably know, is "from now on." It is marked by -no particle because it is meant to modify a noun phrase: either "boku" or "boku-no ikiru shirushi."