[Pitchwife]

Is it Húrin?

Faithful I - his epithet Thalion, the Steadfast, and his resistance to Morgoth's attempt to win him over;
yet I my lord betrayed
To serve the one who wielded shadow - "Turgon, Turgon, remember the Fen of Serech!", giving away the location of Gondolin to Morgoth's eavesdropping spies;
Shadow I helped defeat. - hmm, maybe by covering Turgon's retreat from the Nirnaeth so that Eärendil could be born and raised in Gondolin?

Patient was I - sitting on Thangorodrim all those years, forced to watch Morgoth TV;
but rash move I made.
Madness tore me - could refer to his dealings with the Woodmen in Brethil, or to the affair with Thingol and the Nauglamír;
now at last I
Know what it is to sleep. - finding ease of his torments in death; part of Morgoth's curse was that he couldn't die before Morgoth released him.