Adjective for someone who exaggerates emotions to get sympathy

I would say:

You are so melodramatic.

Because melodramatic means that someone is being overly emotional. See ODO for a full definition.

It doesn't necessarily mean they are constantly reminding people of something tragic that happened to them, but that could be part of it.

Someone who is constantly referring back to a tragic event could be said to be "playing on people's sympathies".

There are many options, each with their own slight difference. Below is a list of the description I'd use, along with the nuance that I would understand them with.

• Hysterical - They are beyond reasoning. They are acting irrationally and cannot be talked down through reason alone (e.g. think of someone who is hyperventilating).
• Dramatic, melodramatic, drama queen - They are intentionally overstating things in order to make things sound as important as they possibly can.
• Theatrical or histrionic - Same as dramatic, but to a degree where they start (over)acting, not simply overstating.
• Attention seeking - This person does it to get attention. Although this doesn't inherently imply being dramatic, it is often inferred.
• Overemotional - This person experiences their emotions more strongly, but they do not intentionally do it.
• Pathological liar - Someone who lies without remorse. Anything they say can be a lie that they tell you to get a response from you. You cannot trust this person to ever tell you the objective truth about anything.