Formal:
* Merci mon amour. (This is the most straightforward and polite way to say it.)
* Je te remercie, mon amour. (This is slightly more formal than the first option.)
Informal:
* Merci beaucoup, mon amour. (This is a more affectionate way to say thank you.)
* Merci mon chéri/ma chérie. (This is very affectionate and can be used with your partner.)
* Merci mon cœur. (This is a very sweet and romantic way to say thank you.)
You can also add a compliment or expression of gratitude after the "thank you" to make it even more meaningful. For example:
* Merci mon amour, tu es toujours là pour moi. (Thank you my love, you're always there for me.)
* Merci mon chéri, tu es tellement gentil. (Thank you my darling, you're so kind.)
Ultimately, the best way to say "thank you my love" in French is the way that feels most natural and authentic to you.