Lying in a relationship

I always seem to lie when I am in an uncomfortable position. I say whatever I think will make the situation easier in the moment. This led to me losing my girlfriend of three years because she could not stand my lying anymore. I have tried to end my habit, but I always seem to mess up. I have trouble committing to things in general. How can I be courageous enough to tell the truth in all situations. How can I stay committed to the changes that I want to make? I want to better myself, but I feel like I lose drive shortly after a commitment. How can I consistently keep promises that I make to myself and others?