Yes, satan understands eternity, at least I think he does. he understands it at least better than we do. While the evil one is not eternal (God is the alone in the category of truly eternal, without beginning or end), he’s certainly been around long enough to get some perspective on it.
his moment of glory came at the cross where he bruised the heel of God. his moment of truth came sometime around daybreak a few days later when he realized with a handful of disciples that has changed hundreds of millions of lives since; a heel-bruise is not a death blow. The Lord lives and conquered and conquers and the outcome of the evil one is already determined. he knows he has lost.
he knows he has lost so now he is giving his all to distract God’s People, to discourage God’s Servants, to dissuade God’s purposes, to defeat God’s Men who proclaim Freedom. satan will do all he can to give you anything and any reason to help you stop speaking, quit loving, sink your passion. he wants you to feed self so he can torment you for eternity.
I do not know what kind of Sunday you had but I do know as a servant of God satan would like nothing more than to distract and discourage you. For if he can he gets you, your ministry and possibly many you might have or will influence. You are a valued prize. Remember, God loves you - the evil one just wants to use you. he hates your soul. If its a tough time I pray for better days. But good days or difficult, your service to God matters. You matter. What you do matters.