Well, surrender has more than one meaning/connotation.
As I said, THAT kind of surrender was alien to Ennis's character. But there is also such a thing as surrender to love or surrender to fate.
I think that if Jack taught Ennis anything in death, it was that he (Jack) lived a life of total surrender to his dream -- which was a dream of love. It is precisely THIS form of surrender that Ennis understood in the end, and he is a stronger (not weaker) person because of it.
I don't mean to be bringing out the dictionary all the time, but we seem to be caught in some semantic interpretation web lately.

1. to yield (something) to the possession or power of another; deliver up possession of on demand or under duress: to surrender the fort to the enemy; to surrender the stolen goods to the police.
2. to give (oneself) up, as to the police.
3. to give (oneself) up to some influence, course, emotion, etc.: He surrendered himself to a life of hardship.
4. to give up, abandon, or relinquish (comfort, hope, etc.).
5. to yield or resign (an office, privilege, etc.) in favor of another.
Before I looked up the meaning of surrender, I was going to suggest a different word to describe Jack and Ennis, their devotion to each other.
1. profound dedication; consecration.
2. earnest attachment to a cause, person, etc.
3. an assignment or appropriation to any purpose, cause, etc.: the devotion of one's wealth and time to scientific advancement.
In a sense, I can see what you mean by Jack's surrendering to his love for Ennis, to give himself up to the power of attraction to Ennis...the short leash, the fourteen hour drive each way. But in another sense, I also see Ennis' devotion to Jack, dedicating his life to those few times meeting each year, quitting job after job, losing a marriage, and countless opportunities like the job at the power plant, just to spend time with Jack.
Ennis also surrendered to a life of hardship, and his homophobia, until the revelation at the end with the two shirts, then he gained the power to shake free the self-imposed shackles, and gained strength from his devotion to the memory of Jack's love.
So, yeah, we are in agreement that Ennis was no quitter, and neither was Jack. They loved each other too much to quit. "I wish I knew how to quit you." was meant as "I can never quit...you/memory of you."