Immortal Flame by Kim Loraine

The end is here and the only one who can stop it is lost.
I never wanted to share Gwen, not with anyone, and definitely not with Lancelot. But now that he’s gone and the truth about my past has been revealed, everything is different.
I used to be a king. I lived my life shrouded by a lie. Afraid to take the love I desperately wanted—love from my queen as well as my knight. That fear is the reason I lost both of them. This is my second chance to be the man Gwen needs, to take up Excalibur and stop the apocalypse, and to finally accept who I truly am.
I am Arthur Pendragon, at least, I was. In our past I broke both Gwen and Lancelot. But this time, I’ll be the hero they each deserve.