Dear Scared of Losing Myself,
These are Normal Feelings
What you are feeling may be very normal.
Share your fears with your boyfriend. Chances are he has some of the same fears. Together the two of you can determine exactly what it would look like if you were to "lose yourself" and come up with some actions that you can take to keep that from happening.
You Lose Part of Yourself
Keep in mind though that marriage is a meshing of two people. You will lose a little of your independent, single self, but it will be replaced by your dependent, married self. If you maintain too much of your independence, you risk becoming married in name alone.