I think it would be better for you to tell the truth, excuses get you nowhere and you can end up losing your girlfriend. If she really likes you, she won't care that you didnt, just your company will do. If you feel you need to make excuses, then she isn't the right girl for you. A good relationship means never having to make excuses or lies.
If you keep on failing at everything you do, don't view yourself as a failure. For you succeed at failing. So to succeed, one must fail at failure!