Based off of a song that I think is called "Locked Away" By someone I forgot *Edit: Thank you to @fan-fiction-4-life for letting me know that the song is by R. City. And thank you to R. City for making an amazing song.
Basically, this super short, really crappy, only-the-second-time-I've-written-for-him reader insert for Tony is OOC but about you dealing with his insecurity of your relationship. By the way, you are married to Tony in this fic.
P.S. If you want to make it more angsty, you can take the "If I got locked away" line as a nod to Civil War and his worries for what might happen to him by the end of it. But you know... Your choice.
I hope you like it! :)

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The whole night sparkled. Sparkling champagne, sparkling chandeliers, sparkling jewelry and cuff links, and sparkling night skies. A life you never thought you'd be able to taste, let alone marry into.
"You okay, babe?" Your husband, Tony, wrapped his arm around your waist and pressed a swift kiss to your cheek, pulling you out of your thoughts. You smiled back at him and replied, "Yeah. I'm great actually."
"Good, good. Look, the press wants another photo op with us and I said only if you're up for it. Something about us being a 'power couple' or something crazy like that." You laughed in response and allowed Tony to lead you over to the press for more sparkling pictures.

You lounged on one of the fancy, leather couches in the mansion you shared with Tony, flipping through a magazine while still wearing your pjs. Since you didn't have anywhere to be today, you decided to enjoy the peace and quiet of your home. Of course, after a few hours, the large empty house began to feel bare and cold.
You threw down the magazine and sighed, wondering when Tony might get home. Just as you thought about ditching your day off and calling in to work, you felt a pair of hands on your shoulders. You tensed for a moment, but relaxed when you heard Tony's voice ask, "Did you have a nice day?" You let out a deep breath as he began to massage your back and replied with another question, "Shouldn't I be the one asking you that question? Being the domestic spouse and all?"
Tony laughed and leaned in close to your ear, whispering, "You're anything but domestic, babe."
"Oh, really?" You purred.
"I think I'd know." Tony replied and began kissing up your neck and along your jaw, eliciting a small moan from you. You turned your head and pulled his lips to yours, quickly deepening the kiss. To make things easier, Tony jumped over the back of the couch and sat next to you. Soon, you were both tangled in passion, and you didn't feel cold anymore.

You woke up to darkness, the bed beside you bare. Turning your head to the digital clock on your other side, you saw the time, 3:45 am. Knowing where he'd be, you quickly threw the covers off of you and made your way to the basement/garage.
"Another one?" You stood at the door, staring at Tony as he tinkered with some kind of machine. His head snapped up to look at you. In reply, he dropped his tools and rubbed his hands over his face.
"I'm sorry. I-I didn't want to wake you."
You made your way to him through the cluttered garage and put your hand on his arm. "I won't mind Tony. If you need help, you can tell me. I want to help you."
Tony stared off for a bit and you led him over to a bench. It had probably been hours since he slept, and you were worried for his health, as always.
As soon as you sat, Tony seemed to shake out of his daze to stare at you. After a moment, he suddenly asked, "Do you love me?"
It took you by surprise for a moment, but then you replied with complete confidence, "Of course I do, Tony. I married you after all." You smiled encouragingly, trying to pass off the situation as a joke. He had to be kidding. He knew you loved him more than anything. He had to.
"You love me? No matter what?"
"Yes," you reassured him, placing your hand on his cheek and stroking it with your thumb.
"So you didn't marry me for my money?"
You sat back in shock, quickly replying, "No. Of course not. Tony, money is nice, but I didn't fall in love with it. I didn't promise to be there for it in sickness and in health. I married you, not your money. Because I love you."
Tony was silent for another moment before taking your hands and staring intently into your eyes, pleading, "So promise me this. Promise me right now. If I got locked away, and we lost it all today. Tell me honestly, would you still love me the same? If I showed you my flaws, if I couldn't be strong. Tell me honestly, would you still love me the same?"
Desperately, Tony waited with bated breath for your answer. Without hesitation, you replied, "Yes. Yes, Tony. I will always love you with all of my heart. No matter what." All anxieties seemed to leave his body in the one swift second after you answered him. He grabbed your face and kissed you, glad to feel your presence and to know that you wouldn't leave him.
After you both broke away, you smiled at your husband and took his hand. You stood up and tugged on his arm, saying, "Now let's go to bed."
Tony allowed you to lead him back to the room you shared, feeling better now that he knew you were his rock and anchor.