When a real hardship crushes me, my prayer suddenly fills with tears and presence, but once life is easy again it goes flat. Must I be in pain to pray with a living heart?
This feeling is not a flaw in your faith. In hardship your prayer fills with tears not because pain is holier than ease, but because pain clears away the thousand small distractions that comfortable days let us hide behind. The heart was always poor before Allah. Hardship only made its poverty audible to you.
This is why Allah calls prayer heavy, and yet light for a certain kind of heart:
“And seek help through patience and prayer, and indeed it is heavy except upon the humble of heart.”
Qur’an, Al-Baqarah 2:45 · meaning
Notice that the burden does not lift because the trial ends. It lifts because the heart softens into humility. Allah also describes the very pattern you are asking about, the calling out in pain and the forgetting once relief arrives, and I will not dress His words as comfort when He sent them as caution:
“And when harm touches man, he calls upon his Lord, turning back to Him; then when He bestows on him a favour from Himself, he forgets what he used to call upon Him for before.”
Qur’an, Az-Zumar 39:8 · meaning
See where that warning falls. It falls on the heart that receives its ease and walks away altogether, not on the heart that sits, as yours does, aching that its prayer has gone quiet. That this grieves you at all is the sign that you have not forgotten Him. And under the warning there is an invitation: turn homeward while nothing is forcing you to. That is what our Master, the Prophet, peace be upon him, showed us. No hardship was crushing him, and still he stood.
“The Prophet, peace be upon him, would stand in prayer until his feet became swollen. When asked why, when Allah had already forgiven him, he said: Should I not be a grateful servant?”
The Prophet (peace be upon him), Sahih al-Bukhari (4836) · meaning
His prayer overflowed from thankfulness, not from pain. So you need not wait for another wound to feel your heart again. Your neediness before Allah is not a visitor who comes only in the storm; it is the constant truth of you, in ease exactly as in trial. The work, then, is to remember in your comfortable days what you already know in your hard ones, that every breath in your chest is borrowed from Him, and to stand as a grateful servant and not only a desperate one.
One small step: Before your next prayer in an easy moment, name in your heart one blessing you would beg for if it were suddenly taken, and enter the salah thanking Allah for it, so gratitude gives your heart the presence that pain once did.