Question for vegans about wearing leather?

What are your opinions on wearing leather, specifically things you have purchased before you became vegan? I have always worn very little leather, and no fur, but I have a pair of Ugg boots which are real leather and I think maybe sheep fur? They were a gift a few years ago and I know they cost my mum a lot of money. I feel guilty wearing them, but at the same time, I think I'd feel worse if I knew that an animal was killed for them and they weren't being used anymore. I certainly would not buy more animal clothing products now, but what do you think about this? Am I a terrible vegan if I keep wearing them? We have super cold winters!