Forgot to say Havdallah and ate. I did say the Omer. What can I do to repair?

Shalom, Thank you for your question. You are correct in thinking that it is forbidden to eat before Havdallah (I’m not sure what you mean by having counted the Omer – but in any events you did count it, so that is good). If one ate before havdallah, they still need to make havdallah. So, the first thing you should do (or have done) is to make havdallah. (If you recalled only on Sunday morning, you still need to make a short form of havdallah). Next, you need to “do tshuvah” – that is repent. Regret, accepting not to fall to the same mistake again in the future, and saying (in prayer) to Hashem a verbal admission of your sin and regret. The best form of real tshuva is to strengthen yourself not to sin again. This can be achieved by both (or either) spending time to learn the laws of havdallah (see here in Rav Melamed’s Peninei Halacha - 08 – Havdala and Saturday Night – Peninei Halakha (yhb.org.il)) and also preforming havadallah with special care and honor in the future. I would like to end with a special note – all of us make mistakes from time to time. That is part of being a person, and what Hashem excepts of us. That is why He created the process of Tshuva for us. The main thing is not to let your spirits fall. To know that one can (and must) continue onwards with greater strength and happiness. Blessings.