Hello,
I was cheated on by my xGF in my previous relationship. I hope it's ok with you if I post.
First of all, I truly admire your courage and strength to come here. It shows that you do care a lot for him and that you do have lots of remorse for your actions. Please, bear that in mind no matter what happens in the future, and know that this shows how much good there is in you.
Secondly, with regards to your question - my (now ex)GF waited for several months to tell me, and I really wished she had told me immediately. And if she had, I think we would have stood a better chance at eventually reconcilling.
Because she was able to keep it inside her for so long and keep me from suspecting anything, it made me think that she could easily lie to me again if it happened again. If she had told me immediately, it would have been better.
Also, I felt manipulated by her not telling me sooner. As if she wanted to wait while I'd become more invested in the relationship, more in love, so that when she would tell me, it would make it harder for me to leave. And it did. And that's why I stayed (she eventually left me, in part because she was unhappy with who I'd become after learning of her cheating). But I really wish I hadn't been manipulated into staying. Especially thinking what if she got pregnant before she told me or perhaps even tried to get pregnant without telling me.
Another reason is that I had had bad experience with STD scares with a previous girl - I almost got involved with a girl who later found out her ex had HIV. Since then, I had been really careful when it comes to this. So this ex, that cheated on me, let me sleep with her for several months after cheating on me, potentially exposing me to life-changing/threatening STDs by not telling me before we had sex again after her cheating. It infuriated me and made me think just how little she cares for my well-being and how much she put her own desires (not to be alone, to me not leave her) before my own desires.
On that note, please get tested for STDs... Please, for your own well-being. You seem like a basically really great gal and I would hate for anything to happen to you in this area and to not be treated immediately.
One more thing - when we started dating, I had told her that whatever happens, that she better never lie to me, that to me that's the worst thing she could do. If she had been forthcoming immediately, I could have easier convinced myself to not only physically stay, but to eventually take the emotional plunge into trusting and openning up to her.
Also, I think when things settle down a bit, you will be glad you have told him immediately, and you will be spared of the guilt of knowing you had kept him in the dark for so long. That might go a long way into forgiving yourself (sooner/easier) than if you compound this bad choice by making further bad choices. And at least it won't eat you inside every day, knowing that you're keeping this big secret from him. Since you seem very remorseful, I imagine it will be only a matter of time before it will have dire consequences for your relationship, but at least if you tell now, it will be the least bad option.
And I think other people have posted valuable insights, and I particularly agree with wheredoigo's post, which was as beautiful as it was sad/painful to read. If she had told me sooner, there would have been a better chance at me staying and doing whatever I could to protect her and soothe the scared girl inside of her. But because she had waited so long, it was no longer us against the world, but me against her&the rest of the world...
In summation, I think if you tell him immediately, you have a much better chance at 1.) eventually being happy with yourself&forgiving yourself and 2.) eventually being really happy, without such manipulation and secrets and with him making an informed decision about staying.
You made a horrible decision. It's time to take responsibility, show your BF the respect and concern for his health&happinness he deserves and not make further bad choices by keeping it a secret.
And personally, if she would have done that immediately, I might have not stayed with her, but I think I would respect her greatly for, after making a bad decision, doing the right thing by me.
Btw, when you tell him, be very honest about the drinking&cheating being your selfish decision. Yes, the loneliness, potential self-esteem issues etc. provide a/the context for what happened, but ultimately you decided to get drunk knowing it was a bad decison. So be very clear on this, tell him you won't drink at all anymore, will search out individual/group counselling to get to the bottom of all this, stay away from guys, definitely stay away from this guy and not have any contact & willing to do other necessary things to make this right. And that you will offer him maximum understanding and support while he deals with the fallout from this. That's what I would have wanted to hear from my ex gf, and hopefully it would help if you tell your BF. (p.s. tell him (and I hope you do) you will do the no drinking & counselling regardless if he chooses to stay or not. That you're doing this not only as something to keep him/you two, but for yourself as well). Oh, and I'd encourage him to join an infidelity site to help him encourage him and work through this. Not this site, but perhaps talkaboutmarriage.com . It would help him and I think your relationship odds as well.
I truly truly wish you and your BF all the best, both individually and as a couple. And I really hope that eventually you will be able to share with us on how great things have been going!