Yes it could possibly be a mortal sin.
Will I be offending God if I quit my priesthood and marry a lady I have dated for so many years and love so much and loves me so much?
Why would God care about such a small thing. Think about how many people he has to watch over. You think he will care about a small minor thing like that. After all he gave you that choice anyways.
Not at all. You are doing the right. Love is more important. Priest hood -think that as a service and you are quitting the same. Wish you all the best.
The apostle Paul said: "we have authority to lead about a wife, do we not? (1 Corinthians 9:5)