Formal/poetic:
* Любимый(ая) моего сердца (Lyubimy(aya) moyego serdtsa) - This literally translates to "Beloved (masculine/feminine) of my heart" and is a very formal and poetic way to say it.
Informal/affectionate:
* Любовь моя (Lyubov' moya) - This means "My love" and is a very common and affectionate way to address your lover.
* Мое сердце (Moe serdtse) - This means "My heart" and can be used to express deep affection.
Other options:
* Ты — мое все (Ty — moe vse) - This means "You are my everything" and is a very romantic expression.
* Ты — моя жизнь (Ty — moya zhizn') - This means "You are my life" and is another very strong expression of love.
Ultimately, the best way to say "Lover of my heart" in Russian depends on the specific context and the tone you want to convey.