Thank you for reaching out to us.
We are sorry that you are so upset with yourself. Sometimes these things happen, and then we learn from them. (As yoatzot halacha, we hear of many such cases.)
Unfortunately, this is too late for the hefsek taharah to count for today. The exception would be if you performed a bedikah (or possibly something like a bedikah) at some earlier point. Please get back to us with details if this was the case, including if there are additional factors in play (e.g., shalom bayit or fertility concerns).
In the future, we recommend performing a bedikah in the morning on the day a hefsek taharah is planned, so that it can serve as a back up after the fact if something like this happens later in the day. Setting an alarm for abut fifteen minutes before shekiah, halachic sunset, can also be helpful.
Please don’t hesitate to get back to us.