Some sweet things to say works like two edged sword. However, if she knows your past, and you (more or less) know her past, feel free to tell her: I’ve fallen in love so many times… (yes, here you make a long looonnng pause – it will feel as eternity for her) and always with you.
Even when she knows you had a lot of girlfriends in the past, she will be happy to here these sweet words. Everyone needs to feel loved, however sometimes you must confirm that by saying sweet things.