yesterday my gf hb10 of 9 months told me that she got drunk and cheated on me with some random dude at the beginning of our relationship before it was serious and it took forever to get her to tell me yesterday cause she was reluctant to say it, in fear of losing me. The shit that pisses me off the most isn't that she cheated it's that she hid this lie for 9 fucking months before she finally told me and i didn't suspect a thing -_-. She didn't even tell me when we told each other that we were in love. I am a really confident guy and this shit hits my self esteem hard out of the many females I've been with as far as i know i've only been cheated on twice including this time. I don't want to break up with her I still love her but what is the best action to take in this situation.

She did tell you, that means its been eating at her for quite some time. But that doesn't make it all okay. Fact is she still cheated on you and if you just let it slide shes going to think that is okay. If this were me I'd go NC for a good while. When/If you decide to break off NC, I'd let her know you wont tolerate this kind of shit EVER again. Make it clear that you don't want a relationship with someone who is going to treat you this way, so you wont take it once more. She was drunk, but she likely still knew what she was doing. Being drunk doesn't kill your conscience. If it was me id honestly actually just break it off right away, but you say you still want to stay with her so in that case that's the advice I have.

Very good advice from viceman. I'd even tell her you appreciate the honesty but you need to think about this for a while. Then go absolutely NC for a week or 2. AFTERWARDS tell her the only thing that saved her was her honesty and If she does it again, obviously it'll be over.
Ideally what you want is letting her experience the feeling of losing you, and trying to break NC.

Dude, I really think you should break up with her, even if you love her. Think what this could do to your self-esteem in the future... You are with a girl who cheated on you!!! If I were you I would just move her completely out of my life.... I can never accept to stay with a girl who cheated on me, but then again I have too much pride. Its your choice in the end, but I advise you to break up.

Depends on what "cheated on me in the beginning of the relationship" means. If you mean, you were dating but not official, you can try to trust her as you weren't exclusive. If you were official, break up IMO.