I hope the reason you have to stop isn't a major tragedy.

A friend of mine had one child and had to stop trying for a second due to cancer. For her the switch to fighting for her life dwarfed the decision to stop trying. It certainly gave me some perspective to hear her story.

Time will help, but it will have to be a lot of time and you'll need to get the clock to start ticking by moving on, even though it's painful. The act of giving away stuff forces you to start that journey. Re-imagining your life with 3 and thinking of things you might be looking forward to.
I liked to think of cool vacations we might get to go on sooner.

OP, I have given stuff that I really liked to friends and saying basically, we're done, but if for some reason the universe changes course, I'll grab it back! That has really helped me get rid of some stuff.

giving away stuff doesn't really make anything final. you can always buy stuff. as somebody said, start with the bulky items first. i got rid of the tub, car seat, i sold the beaten up/dirty stroller.. eventually had another baby and very much enjoyed buying her new stuff. i never missed the dirty stroller or the old tub.

i don't know your circumstances and i am not saying you should hold on the hope but regardless of whether you keep the hope or not, you don't need to turn into a hoarder. even if you had five kids, eventually you would need to get rid off the baby stuff.