How do I get my girlfriend to forgive me after I made a huge error

[Thetruedude]

I really love my girlfriend, but I went running to my friends talking about all the bad things about her, because I get stingy when she’s corrects or bad mouth me, and never told them about her good things. I know what I did was wrong, but i don’t know what actions to make in order for her to forgive me.
I tried phone calling her tonight, but all I could say was “I’m sorry” and gave so many excuses as of why I acted this way

[Zinok]

Wait, we need more information.
How did she find out you were bad mouthing her to your friends?

Also, that's a shitty thing to do. I get it, you want to vent when things are bad, but you need to build her up and be respectful too. Even when things are bad and you're venting to your buds, be respectful. This is a girl you claim to love, so act like it.
And what "excuse" is there to treat her that way? Maybe you should stop making excuses, that completely negates the apology. What you did was shitty. No excuse. It was. You fucked up bro. If you're trying to justify it to her, why would she ever forgive you? Not cool.

[Sandra]

You need to take it a step further to ensure you can repair the relationship and get her back to feeling close and connected to you again.

When it comes to saying sorry to your girlfriend you have to do it with meaning and conviction. Depending on what you did, this may mean that you need to apologize several times or just once. Although it is tempting to accompany the apology with a gift or flowers this actually isn't the best idea. It can be misinterpreted by a woman if she's feeling highly emotional. She may see the goodwill gesture as a bribe and the sincere apology you were trying to deliver will be lost in translation. Leave all gifts out of it. Instead ask her to meet with you and then tell her you're sorry.

Trying to explain why you did what you did should come at a later time. When you're apologizing to your girlfriend you want to keep it simple and straightforward. Tell her that you regret what you did and you deeply regret the pain it has caused her. Let her know that you are just as disappointed in your own behavior as she is.

The next part of your quest to get your girlfriend to forgive you is to show her that you truly are a changed man. Telling her that you'll be different or you won't repeat the same mistake may be lost on her. She may doubt that and it would be for good reason. Your past behavior is what is guiding her decision making now so you need to show her, through your renewed actions, that you're a better person who has learned from his past mistakes. Change whatever behavior caused you to hurt her. Let her see that you have a renewed purpose in life and that's to make her trust you and your love for her again.

Obviously it will take time, but with patience and persistence you can rebuild the relationship so it's even better than it was before. You love her so she's more than worth the effort.