I find, when struggling to put into words how much I love my partner, that it helps to create a list of the specific thing I love about them. It doesn't have to be a list of extravagant things. I think people like to be given compliments about things other than their appearance, though it's always nice to be told if you're looking good!
After the first time, it should get easier to say that you miss him. There are, however, other ways of saying it without using the actual words. For example, you could say 'I wish you were here' or 'I wish we could spend more time together', if those are easier for you. Hope this helps.