I found a website that has idioms for you to look at! http://www.idiomsite.com/ and I found an article about idioms and how they are in arabic (it requires PDF) http://hmaxos.com/Arabic%20idioms.pdf

Well I am not experienced in love but from what I see on the sidelines... if you haven't said you love her before then it may be awkwardif you randomly say "I love you" after all those three years. If you truly love her then I think it'd be nice if you set the scene when you two next go on a date somewhere where there's not a lot of people and tell her.I don't know anything but just say a lot of things about happy times in the past and then say "Basically it means I love you". Hahaha! I know it's very cheesy but atleast you have my answer. An idiom is a phrase in which native speakers or people who come to intergrate with the country will understand like "Killing two bird with one stone" means to get two things done by one action. Also, "Back to square one/the drawing boards" means when something doesn't work and you have to start again. I hope it helps, that is my understanding of idioms!