Formal:
* J'aime mon/ma partenaire. (This is the most neutral and formal way to say it)
* J'aime mon/ma bien-aimé(e). (This is a bit more romantic)
Informal:
* J'aime mon/ma chéri(e). (This is a very common and affectionate way to say it)
* Je t'aime. (This is the most direct and intimate way to say it, but you can only use it if you are already in a committed relationship)
Note:
* You will need to use "mon" (my) for a male lover and "ma" (my) for a female lover.
* "Bien-aimé(e)" means beloved.
* "Chéri(e)" means darling.
* "Je t'aime" literally means "I love you".
It's important to choose the right phrase based on your relationship and the situation. If you are unsure, it is always best to err on the side of formality.