SUKI does have both meanings, "to like" and "to love". There IS a word, "to love" in Japanese (AISHITEMASU) but we'd rather use SUKI DESU. I don't know why exactly. Maybe because we like to say things indirectly, we say "I like you" instead of "I love you". Or maybe we are not romantic enough to use the word... If anyone has a better idea, let us know. You can also say "DAISUKI DESU" (=I like you a lot), which is a little more aggressive way of "I love you" in Japanese. I guess... I occasionally hear people say AISHITEMASU or AISHITERU in the movies and dramas. And I think some grown-ups do use these words in some situations. But compared to SUKI DESU, AISHITEMASU is such a heavy word, and not easy to use. I wonder if it's the same way in other Asian countries...
I would like to add something related to the topic. I often hear this question from my students. We don't say "I love you" (AISHITERU), not even "I like you" (SUKI ), to our family members as you do in America. We don't hug or kiss them either unless they are small children. Even between boyfriend and girlfriend/husband and wife, they generally don't show affection in public.
I don't think I'm around enough couples to really notice what they say but my girlfriend and I use "aishiteru" or "I love you" often enough. I am American and she is Japanese. She doesn't have any problems at all telling me that she loves me and often at that. Perhaps because I am a foreigner?
I think that the reason you and your girlfriend can say it to each other easily is indeed because you are a mixed couple. My wife is Japanese and we are like you guys. But like Yuki said, Japanese culture makes most Japanese embarrased to show affection, not just in public actually, but in all situations. Even in the house most Japanese couples act like they have a domestic arrangement, not a romantic bond.
The question of the difference between suki and aishiteru is interesting because it's one of those things you can't explain based on the standards of the opposite language. Suki could be roughly equated to like and aishiteru roughly equated to love, but because the Japanese language is based on Japanese culture and Japanese people do not like and love things in exactly the same ways and situations as English speakers do, the words cannot truly be matched up one-to-one.
I guess I'm biased because I'm an American but I really respect the Japanese who stand out from the rest of their fellow Japanese and show love and affection to their children and spouses without embarrasment, but with true feelings.
Again Ok I actually brought this issue up with my girlfriend and tried to get an explanation out of her. She told me that younger Japanese couples tend to use "aishiteru" a lot more often these days. She says that older, married couples tend to not use "aishiteru." She said that many of her friends, who are Japanese couples, often use "aishiteru" though. She said though that it is obviously reserved for very serious relationships when people really do love each other. But apparently it's used a lot more these days than just a few years back.