Formal:
* 我想你，我爱你，亲爱的 (Wǒ xiǎng nǐ, wǒ ài nǐ, qīn'ài de) - "I miss you, I love you, sweetheart." This is a very direct and formal way to say it.
Informal:
* 我好想你，我好爱你，宝贝 (Wǒ hǎo xiǎng nǐ, wǒ hǎo ài nǐ, bǎobei) - "I miss you so much, I love you so much, sweetheart." This is a more casual and affectionate way to say it.
* 我很想你，我爱你，亲爱的 (Wǒ hěn xiǎng nǐ, wǒ ài nǐ, qīn'ài de) - "I miss you very much, I love you, sweetheart." This is a more affectionate way to say it, but still relatively formal.
Slang:
* 我好想你，我超爱你，宝贝 (Wǒ hǎo xiǎng nǐ, wǒ chāo ài nǐ, bǎobei) - "I miss you so much, I love you so much, sweetheart." This is a very casual and affectionate way to say it.
Tips:
* "亲爱的" (qīn'ài de) and "宝贝" (bǎobei) both mean "sweetheart", but "宝贝" is more informal and affectionate.
* You can use different words to express different levels of missing someone, such as 想念 (xiǎng niàn) for "missing" and 思念 (sī niàn) for "longing for".
* You can also add other words to make the expression more specific, such as "很想你" (hěn xiǎng nǐ) for "miss you a lot" or "非常想你" (fēicháng xiǎng nǐ) for "miss you very much".
Ultimately, the best way to say "I miss and love sweetheart" in Mandarin Chinese is to choose the expression that feels most natural and appropriate for your relationship with the person you are speaking to.