Britt it is late in the year and that mares cycles will not be as long or always 21 days apart.
You need to get that mare an ultrasound.
I just took two mares to be checked after they came back from the breeders and the cost for both was minimal less than 150$.
Take this anyway you want to. It is meant to be advice if you are offended that is your problem.
With your limited resources and knowledge you need to take every precaution possible to avoid costly complications. Not to mention learn more about breeding horses not just taking chances.
You have an older mare that is not an easy keeper that is two strikes against her. How many more are you willing to overlook?
Britt I am certain you mean well but an open mind is an asset not a liability. I am pretty sure if something happened to that mare you would be emotionally devastated. Are you willing to add guilt because you refused good advice?
Good luck. I mean that . Shalom