Considering it will be the very first time you will say it which one is harder to say, "I love you" or "I am sorry"? In which situation will it be the reverse for you?
Good point Daishain. I think they are both difficult and for different reasons. Saying I'm sorry to someone you've said I love you to should be the easiest thing in the world… and yet it often isn't. The times it isn't is when you have grievously hurt that person and the reason it is difficult to say is that you have to admit to yourself the wrong you have done to someone you love.
As for saying I love you, there is the point Daishain pointed out very well. No need to add to that. There is also the fear of rejection or making oneself look foolish. And then there is the very real fear saying something too soon and scaring away your partner. Lastly, there is the vulnerability you open yourself up to… because once said out loud it becomes even more real, and that leaves yo vulnerable.