Not completing a practice

Of the 3 fields, the swimming is the hardest for me.

I don't know if it's because I am a big person (192cm, 120kg), or simply because of the reason that you can't just stop and rest.

I have a feeling that my strokes are not strong enough, or whatever, but last night, I could not finish my practice. I went to swim at 1900, but did not complete the entire 2k.

I am not sure it's a good thing, or bad thing, but I hope that this will gradually stop in the future.

Thanks