Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
These past few [time interval] have been wonderful. My feelings for you have grown so immensely, I now understand what they mean by falling "head over heels". Certainly, you know how I feel about you. I haven't tried to hide my feelings. What may surprise you is that you are beginning to feel the same way about me.
It is not my intention to make you uncomfortable, quite the contrary. These are probably new feelings for you. They might even be a little scary. Please, don't worry about where we are going. I care for you and I know you care for me. That is all I need for now. So, please take your time and let's see where this relationship leads. Let yourself get comfortable with these feelings, I know when you are ready you will tell me how you feel.
Yours truly,