Write her a letter. Obviously you have issues expressing yourself when you're put on the spot, so write it all down and put it away for a day. Take it out, read it, and rewrite anything that doesn't feel right. Put it away again, take it out the next day and share it with a friend or on here and make any recommended updates. Then sleep on it. Make sure it's something you can live with. Then either give it to her directly or leave it somewhere she'll find it.