1. I will love you forever because you are the love of my life! I love you not casually.

Generally speaking, if foreigners want to express unique or lifelong love similar to what we say, they all use If You Are the One or My Forever Love. They seldom use their only love.

So ...

The first sentence:

I will always love you (because this sentence is actually a lyric, which will be more romantic for foreigners), because you are the only one for me .../because you will always be my love. ...

The second sentence:

I love you, and that's not just talk. ...

I love you, this is not something to talk about.

I'm not joking with you, and I'm not talking casually. It's a fact. I'm in love with you, and this love will last forever. ...

Extended edition.