Mental strength during training

"want it"?

For me....I "want it". If you're going for a full, I'm assuming you've probably already got a good deal of fitness under your belt and maybe don't have the same rewards I had. I didn't have significant fitness experienc4e when I signed up for my first sprint. When I started training for a half, the plan eventually called for 9 miles....then 10.....then 11...12...
Being that I'd only run 9 once before (kinda by accident) and never more than 6 a couple half dozen times......

Achieving those distances was a prize in and of itself. Granted, the last few weeks were tough. Lots of longer sessions. Anxiety and anxiousness kicking in too. Both frustrating and motivating at the same time.

What desire to quit? The only time I stop racing or training is to prevent an injury.

I know exactly what you mean. That's why I will never be able to train for longer distances than sprint, which I am happy with.

I don't know how to help on the swim, given the setup, but on a run or bike you can have someone drop you off the amount of miles from home, and then you have to finish the workout or else you don't get home!

I always lie to myself that "ill only do a short run (just to get out the door). maybe 4 miles." Then after a few miles I start feeling good and then change my game plan "yeah I can do 12mi" and typically bite off more than I can chew. Then when the going gets tough around the later miles I have no choice but to grind through the final bit to get home. Endorphins kick in at the finish and I'm primed for the next time.

this doesn't work in a pool. in a pool may I suggest a swim partner. I know that im less likely to quit early when there is someone else there keeping me honest haha.

I'm kinda the same way with running/swimming (cycling is different....I'm more likely to add miles!)...except I'll go out fully intending to do X.
At some point I start thinking X-Y sounds good. But, I find almost EVERY time that if I say...."Fine, X-Y it is...but only if you go X-Y+1."....that once I do that +1...I'm good to go to complete the full X.