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Tony Stark
He feels that way more than you think yourself, so he knows what is the best way to make you feel better. If you're not willing to talk, he's ready to hug you, since you feel like his own kid to him. He held you there until your tears stopped flowing, and let the matter you were having slowly sink in. It needs Tony a lot of patience and time to make you feel like you're enough again, but eventually, he succeeds in it.
Thor Odinson
If someone made you doubt if you're doing enough, he'd tell them to apologize, but if you had failed in something and needed comfort, he'd just come over and talk to you until you change the subject to something happier. He'd talk to you as long as you wanted, and in the end, he'd told you that you are in an important part of helping the Avengers, and without you, they wouldn't manage as well as they did now.
Natasha Romanoff
The old Natasha maybe wouldn't have known how to comfort you - but the one she had grown into had the skills to just come over you and hug you. Let the tears fall on her shirt while she was whispering things like "it's okay, cry as long as you need to". And when you tell her that you feel like you should have done more and that you're capable of doing only nothing, she just shook your head and told you to listen. She told you all the times you had saved someone while on missions, and she made you gradually feel better.
Bruce Banner
Bruce would talk to you too, and maybe show some physical affection if you cried. He'd offer you tissues and someone to talk to. He's not as good at comforting as some of the other Avengers, but him listening and occasionally giving praise for your actions or comforting words during your talk, helped more than you wished for. And fortunately that was enough to make you believe that everything would turn out for the better in the end.
Steve Rogers
When he sees you blaming yourself and saying that you should have been able to do the impossible during the battle, Steve remembers how he realized he isn't going to see Peggy again - and he kinda sees himself in you at that moment. Like he should have survived from that crash, but in truth, he didn't stand a chance, it was either him or the city. And you had acted for the sake of the other people - and still, some were lost. You blamed yourself for it, so Steve offered himself as the one you could talk to if you needed. And after hours, or even days of comforting you, you finally started to realize that maybe it wasn't your fault, and you couldn't have done much else what you did.
Clint Barton
When Clint sees you collapse down and say over and over again that you didn't do enough, that you aren't enough, he made you sit on a chair, gave you a glass of water and talked about it. He didn't know any other way to comfort you - he certainly couldn't just leave you there. And after crying about the same amount of water than you afterward drank, you opened up to Clint and just talked about it. He listened to you and just by that, he made you feel a lot better. He would say a few things to make you feel better again, like remind you that you've done much more to the Avengers than most, and that you would be able to stand back up again every time you fall. You believed him - and he was right.
Loki Odinson/Laufeyson
With all the other people, Loki would think that losing faith in themselves is a puny mortal problem and certainly not for Asgardians, but with you, he suddenly feels powerless when he sees you being so sad and miserable, since you're the only person besides Frigga he truly cares about. He wouldn't show any physical affection, but he would tell you that you're one of the most powerful soldiers he has ever met, and that feeling like you're not enough is just a thought in your head which should be banished. He tells you that he informs his father that you're taking a day off from your duties and go to rest if you feel sad, and Odin, of course, understands your struggles.