It's possible. It's not easy.
20 years is a long time.... Depending on the types of hurt and damage you and your drinking caused, it will require a lot - of both of you.
Your job is to stay sober and make it known you want to be the man she deserved all these years and you'll give that everything you can, that you want her forgiveness. But that forgiveness won't be easily earned..... You'll need patience and humility and empathy and understanding - and probably a thick skin.
She will need to learn to forgive and trust and let go and process and cope with half a lifetime of wounding.
You both will need support and commitment and probably a skilled and qualified couples therapist who deeply understands the impact of addiction to all involved.
It can be done. I have very close friends who are living proof. It is also a real challenge for both parties, and is unfortunately also rare....
Be open-hearted, humble, give her your pledge and live by it. You can only control YOUR part in this.