Macbook Pro 13" 2017 startup problem

My MacBook was fine until a few months ago. If I close the lid, it doesn't go to sleep I have to physically go and tell it to sleep. This is not the main problem. The screen keeps going black when I restart from either sleep mode or shut down mode. It then tells me the battery is extremely low (an icon with flashing red battery). The only way I can restart it is to plug it in. It then starts up and if I look at the battery it says anything from 50% to 100% charged and continues to show this as I work. I tried the diagnostics (holding down the letter D) but it doesn't work even though I followed the instructions to the letter. This is annoying as it has occasionally gone black screen when I'm in the middle of something and then I have to plug it in and start all over again. Surely a battery should last longer than about 18 months to 2 years?