Resilient Caster

An adventurer's life is hard.  Days filled with danger have bolstered your physical and mental response to attacks.  What sometimes kills you can make you more resilient in the face of death, but at the risk of becoming overconfident.

You gain proficiency in Constitution saving throws.
You have advantage on Constitution saving throws to maintain concentration on a spell when you take damage.
After your first death, add your Constitution modifier to death saves, minus the number of times you have died.