Not one sentence but here are mine;
"What's the point?" (No 1, ever present thought)
"Love is everything and you don't have it"
"You will probably never find love again and even if you do it may take you decades again or turn to shit"
"Yes there'll be good times again…..but well outweighed by bad"
"Just think how many more colds you'll have to catch, shitty illnesses, bouts of crap weather you'll have to endure in a life time…"
"Your body and sex life are going to turn to shit the older you get. Your best years are over."
"Childbirth and breastfeeding ravaged your body and the damage is irreversible"
"If people are that bothered by me leaving, they're welcome to do the same and leave too"
Then of course, there is the fantasising. I see a high bridge or building or railway track and just think "what if I were brave enough…"
My favourite intrusive thought is of being in Peru, slipping myself some N with a nice tune playing, watching a final sunset from a balcony.