This is kind of a natural AIBU but I thought I'd put it here, because it is used mostly in the context of relationships and that is where it does most harm.
"You've made your bed, now you must lie in it." This is one of a number of platitudes trotted out by the older generation (ie anyone who appears higher up than yourself on the family tree) and even some people who are young enough to know better, when they get wind that someone is thinking of leaving their twunt partner. A lot of the sayings I can take issue with in context ("do you want your children to come from a broken home" being one, "families need fathers" another, and "it takes two to tango" being fair enough in some situations but extremely annoying in others). However the bed-making one is the only one that makes no sense at all, either as a metaphor or as straight out fact. I challenge it whenever I see it on a thread, but it's so bad it should have its own thread to shame it and put it to, aha, bed for good.
Think about it. You make the bed one day but for some reason you've made a pig's ear of it and it's all lumpy. You're really tired so you lie down anyway, but you just can't get comfortable. What is wrong with getting up again and pulling the underblanket straight? Isn't that the sensible, logical thing to do? Even if you made it in a literal sense with a hammer and a few planks of wood, if it turned out wonky you'd take it apart and have another go. If the worst comes to the worst and you're really too tired you can go and spend the night on the sofa. Would your mum really tell you that you mustn't sleep on another piece of furniture because you own a bed, even if it's broken? If she did, wouldn't you think she was a bit barkin'?
Now for the metaphorical sense. Supposing your affectionate, supportive, hard-working fiancé turns, after the knot is tied or after the first baby comes along, into a violent, alcoholic cock-lodger, or after vowing faithfulness spends every night out chasing skirt and brings home some lovely diseases. There's all too much of it about. So tell me why it makes any sense that a woman who has been deceived in this way should be expected to stand by her decision? She made a bargain, but he did not keep his end of it. However, because she "made her bed" - ie invited him into it - she is supposed to try to hold up one end of the bargain forever. Please explain to me why she should lie in this bed when the other party keeps messing it up.
Lastly, neither people nor relationships are pieces of furniture. People are much more complex and changeable, whilst a bed does not change unless somebody or something changes it. Plus you can actually chuck out a bed; it's neither so easy nor, usually, decent to do it to a person. Relationships are not a "thing" at all - they're metaphysical, an expression of the interactions between human beings, infinitely subtle, infinitely malleable. You remake a relationship every time you talk to your spouse, every time you call or don't call your parents, every time you cuddle or reprimand your child. It's developing all the time, for good or ill. It doesn't just sit there like a lump of wood, waiting to serve you. You have to service it, nurture it, even when it's easy and nice. It's more like a pet or a plant than a bed.
The point they are probably getting at, or their wise old granny was getting at when she used the phrase to them, was that if you take on responsibilities you have to see them out. That, of course, you do. They don't let you take your baby back to the shop if it isn't sleeping as well as the one you ordered. But it's no bloody help to say "well you wanted to have that baby, tough"; a decent relative or friend would come up with some helpful suggestions, refer you to sources of advice, or even offer to mind the baby for a few hours while you flake out. There's nearly always something you can do to improve a bad situation, and as human beings, we have both the capacity and, I would argue, duty to do so. If the first cavemen had flung down a few flea-ridden furs and said "there you are, that's the bed, lie on it", we wouldn't have sprung mattresses today.
There is no need to suffer in silence. There is no need to lie on that uncomfortable bed. Get up and do something about it, as you wanted to in the first place, and be damned to those shrill voices telling you to lie still and pretend to like it.