How do you know when the time is right to stop trying?

I'm torn, my son is starting school in the fall and I am not going to lie I'm excited over the fact our daycare is going to be reduced significantly. We've been TTC for a couple years now and I'm almost to the point in throwing in the towel. How do you know that's the right decision? I feel torn on the fact of making that decision I never only just wanted one but I'm tired of being disappointed every month. I'm trying to stay positive in the fact that with just one I can give him everything he wants but deep down I do want him to have a sibling hopefully someday