I am a teenager who still sucks her thumb. Do you think the use of an adult sized pacifier would help me?

No. Using a specially designed thumb guard, available without a prescription, should help. Substituting one object for sucking instead of your thumb is not helpful. Behavior training that eliminates the habit, and consideration for counseling to explore the emotional and behavioral benefits you gain from the habit, to help guide you to healthy choices to acheive healthy benefits, those are helpful.
Yes and NO. Stimulation of the lips stimulates pleasure centers, explaining why kissing, smoking, gum chewing, thumbsucking & pacifiers have soothing effect.Cultural expectations push thumbsucking into a covert activity as we get older.Long term distortion of teeth & thumb is problematic.Substituting one taboo habit for another may help in the short term but finding another method of self soothing would be better.