I think what he means by "my heart is my own design" is that his heart cannot overrule his mind.
basically-it doesn't matter how hard you try to tell yourself that "you've found the right girl" if "this feeling is starting to stick" and you "wonder why you make yourself sick" then your heart is telling you that this girl is not the one.
"well it must be assigned" that his heart "is his own design"- his heart overrules his mind. His heart represents who he is, and who he is depends on his own design. He can be whoever he wants to be. He doesn't want to be untrue, his heart is his genuine self -so "keep that in mind, when it's all over" when he breaks up with her or when he dies
I think what he means by "my heart is my own design" is that his heart cannot overrule his mind. basically-it doesn't matter how hard you try to tell yourself that "you've found the right girl" if "this feeling is starting to stick" and you "wonder why you make yourself sick" then your heart is telling you that this girl is not the one.
"well it must be assigned" that his heart "is his own design"- his heart overrules his mind. His heart represents who he is, and who he is depends on his own design. He can be whoever he wants to be. He doesn't want to be untrue, his heart is his genuine self -so "keep that in mind, when it's all over" when he breaks up with her or when he dies