We never have to stop praying. We are encouraged to pray without ceasing. (I Thessalonians 5:17)
There are formal and informal ways to pray. When we accept this, the many ways to pray 24/7 open up to us. We don't always have to get on our knees with a scripted intonation of phrases and quotes.
Prayer, in it's essence, is talking to God. Talking to Jesus. How much more candid, honest and heart-felt are conversational words versus rehearsed prayer speeches.
Being tired or discouraged and not feeling like praying is not even an excuse. We can ask the Holy Spirit to pray for us. He will then utter on-target words that we cannot.
Pray without ceasing and grow closer to God.