Do this for all of your major characters (defined as anyone who helps move the story forward): On a separate document, slip your feet inside your character's baby shoes, and using first person POV have that baby, child, young person, young adult tell the story of his or her life up until they enter your story. It will cure all of the problems listed above in the first paragraph. In addition it will add reality (depth, texture, feelings, events, pimples) to your characters, and provide you with a wealth of background material for whenever your characters get introspective.
No need to include each and every diaper load. For a cue regarding what to include, use your own memory of your own past as a guide: earliest memory, the people you remember, your interactions with them, how you felt about them, first love, first hate, fights, celebrations, whatever was important and revealing.
Why the title, "Growing Up With Thomas?" The first time I did this full bore was with my novel Sea of Glass, whose POV character was Thomas Windom. He is still more real to me than most of the persons I have known for decades. I've also done this with my entire Joe Torio Mystery Series, beginning with The Hangman's Son.
If you haven't grown up with your characters, you don't know them very well. Not knowing your characters is a terrible platform from which to try telling their stories.
Try it out and see the difference it makes both in your story and in your writing day. Yeah, it involves extra writing. But, if you don't like writing you'd best nail down that Burger King gig.