Emotion refers to the physical; what you do, the way you behave...e.g. getting nervous when you see somebody you feel for. The sentiment is the part that can be expressed...e.g. telling that someone: I love you. The feeling is the sensation itself...e.g. excitement

Use sentimental to indicate mood, as in someone who reflects on his/her feelings, usually pertains to soft, nostalgic, and friendly feelings.
Use emotional to indicate someone who reacts to events by expressing what he/she feels.
Sentimental connotes an internalized feeling, emotional an expressive one.