Need help saying I love you

I have a Japanese girlfriend and we have become very close.
I have fallen in love with her and I want to tell her.
I dont know how to tell her in her language.
Also I want to do it in such a way that shows respect to her culture.
Can anyone help?

There are many ways to say it

Ashiteru

dai suki desu.

There are many threads on this forum already, talking about "I love you" in Japanese. If you just want to say a translation of the English words, the above poster has already answered your question. If you want to be "culturally sensitive" then well, it really depends partially on the personality and character of your girlfriend, but in Japan many people never say "I love you" at all. They just show their love by their long-term committment and their actions showing how they care for the person.

1.Ashiteru

2.dai suki desu.

I've seen many posts where the claim is made that aishiteiru and daisuki both mean "I love you," without making any detailed distinction between the expressions.

For me, there seems to be a big difference between the two, but since I'm not a native speaker, I can't say for sure how much a Japanese person would differentiate between them. Can a Japanese native speaker comment? If someone says, "Dai suki!" to you, does that mean that they have fallen in love with you? Or does it only mean that they care about you/ feel fondness for you?

(Just an after note)
dai suki is more commonly used, and aishiteru is something very serious. almost marriage serious. Of course, I am also not a native speaker. so I'm not sure how correct that is. I say aishiteru to my boyfriend, but he speaks english, so he doesn't really know the difference, I just feel better saying it like that.

Also, pronunciation, dai (dye) suki (ski) desu (des)
aishiteru (eye-shi-te-ru)
(nihongo "r" is like a R, D and L all mixed in one.) Just in case you're not sure, although with a nihonjin (Japanese) gf, you probably would.

If you're going to say Aishiteru, don't say it too often! I'd stick with dai suki desu until it's more serious, if it's already like that, go with what you feel more comfortable with, I'm sure she'd feel the same and reply in kind.

Daisuki pronounced Dye-Sue-Key is a very casual way of saying I love you.

Ai shiteru basically means I'm in love with you and it's a very strong way of saying it. Of course if you were to say ai shiteru to her, I'm sure she'd think it's very cute. Normally I don't think a native Japanese boy would say it to a girl, unless the relationship was very serious and he was proposing marriage or inside a marriage.

As a foreigner you can probably get away with Suki or Aishiteru.

As said Aishiteru is not commonly used at all (out side of movies or drama anyway, where it is used a lot). But She'll understand either way... and probably think it's cute cause your a foreigner, she likely won't expect you to have perfect understanding of a very high context culture.

I would recommend "suki desu" as it is more common and not offensive. "Aishiteru" in some cases can be received as sarcasm so I think for serious ocasions "Aishitemasu" can be more apropriate and polite. Personally I preffer "suki desu" and if emphasis is needed you can say "suki desuyo" ("I love you", same as "suki desu" but more emphasised) or "Honto ni suki desu" (I really love you)...Hope you're covered.