Fight Through the Pain is a defense gear perk that raises your general defense after you heal manually. It helps you stay in a fight right after patching up.
After a manual heal, you take less damage from every source for a short time. That gives you a window to push or reposition safely.
Good for players who heal mid-fight. It is weaker if you rarely use manual healing.
First Aid Kit lets you heal more often, and Endorphin Rush adds speed after healing.