I have a bad habbit of lying unintentionally just to show my indifference now it's affecting my relationship.I promised my gf I'd change that fact about me but failed yesterday.we don't want to break up with each other but she was quite enraged by the fact I lied.I want to change so our relationship can head towards a good direction

Take a breath and count to three before you answer things you are prone to lying about.

So first you need to identify the kinds of things you lie about and then consciously counter the behaviour.

You need to get your girlfriends co-operation to call you out whenever you do this without losing her cool because it isn't a behaviour that will change over-night and her support will help you immensely.

It's important she understands that it's an unconscious compulsive behaviour that you aren't consciously doing, similar to things like chewing nails.

OP i've had the same problem as you for quiet a while now. Its not really something that can be fixed easily, hell i still haven't completely gotten over it yet. But at least for me i had to realize that i was lying for a reason, my reason is that i would lie to peoples faces in order to ironically appear as a better person in their eyes and also to just try to get a positive reaction from them. for example if a person was talking about how they liked a new movie that was out i would almost automatically lie and say i liked even though i didn't just to stay on their good side, for no reason at all.

Being proactive against this is very hard, and it won't change overnight. What really helped me out was lets say for example somebody was asking me a question. I know for a fact that what comes out of my mouth is probably going to be a lie, so i would intentionally pause and look upwards kind of like i was actually thinking about my answer, but instead just focus all my energy on telling the truth.

then again thats just me