Mother changed the locks and refuses to give me a key

I moved back home last year due to financial issues with my 2 kids. Things between us has always been pretty fragile and mainly it’s because of my children’s father. Because she hates him, she takes it out on me when I’m telling you right now, he literally does nothing....wrong....we both feel through the cracks and she’s blaming him when we both made some stupid choices that were fixing.

The reason she changed the locks is because she believes he has a copy, which he doesnt. The only time he entered the house was when I gave him permission to grab some diapers and wipes while I was at work and he returned the key. She didn’t believe me and called me a liar.

I pay rent, some utilities and I also pay her to watch my kids when I need her to babysit for 2 hours before I get off. I don’t find this fair. What can I do or is there nothing I can do?