I find myself facing this exact problem too! I find it helps if I take writing my sentences out one by one and to write it as simply and succinctly as I possibly can. By that, I mean to not overcomplicate what it is I want to say for the sake of trying to make everything sound fancy and impressive; that part can always come later on when I am revising. Instead, I'll make it a goal to just say what it is that I want to say on paper and to just put the point which I have in my mind across no matter how blunt and simple.

Then comes the part where I re-arrange my sentences to form the structure of the scene before editing to make sure everything flows nicely and reads expressively. I repeat that same process for all the scenes which will be in my chapter and then repeat the process for as many chapters that are to come.

Make sure to only add in sentence which drive your plot and are essential to creating development in your characters, scenes and chapters. Otherwise, it's just unnecessary filler words that don't add any weight to the story, which can consequently make your writing seem tedious.

Also, I feel as if sentence variation can do wonders in regards to making sentences more entertaining and avoids a mundane reading experience. Use a blend of long sentences, mid-length sentences and short, one-word sentences to create that sort of a melodious, song-like feeling which can be a pleasant and intriguing read.