Let me diagnose you. Do you do any of the following but expect that once they are said that the outcome will be different to the other one thousand times you have said it? If so, I am afraid you have caught 'Repetitive Mom Syndrome':
To siblings: Say "be nice". They won't. They will try to kill each other on a regular basis. They will never be nice. And if they are? It will last for ten minutes max. Till you have to say it again. And again. And again.
To young children with pets: Say "be gentle". Again they won't. Oh sure they will when you are looking but as soon as you turn your head the cats tail will be pulled or the guinea pig will be smashed in the face with a tiny fist.
To children in general (especially on play dates): Say "play nicely". This is touch and go and such is the tricksome world of small children. Sometimes they will. Sometimes they will fight for a full hour over the grubbiest mankiest toy know to man.
To toddlers: Say "put your coat on quickly". Never ever EVER going to happen. Your house could be alight, Mr Tumble could be in your drive, a brand new 3D Peppa film is opening at the cinema. Nothing is going to make your toddler put their coat on quickly. Bar your forcing them into it. Whilst the child screams. **please note unless you don't want them to wear a coat. This will be done quickly**
And finally:...The dreaded word. The most horrible of concepts. "Share!" you could say this till you are blue in the face. You could tattoo it on your face. Sharing is not going to happen. Ever. Embrace it. And just buy two of every toy your child owns.
Unfortunately there is no cure for 'Repetitive Mum Syndrome'. I'm afraid there is only one way to get over it. Denial and blind hope that things will change. They will one day. By which point you will be so beaten down you won't care any more.