Tony-

Your biggest fear is that Tony is only with you for your looks. You really do believe that he truly loves you but that piece of doubt just refuses to leave.

Loki-

You're afriad that Loki will leave you for someone more "worthy". Since Loki looks down upon humanity you are petrified that one day he'll just up and leave you because you're just a simple human.

Clint-

You're scared that despite what they say Clint and Natasha might have feelings for each other. That makes you worry because those two have been through so much together and they are so close already that it wouldn't be that much of a jump to start dating.

Steve-

You think that Steve compares you to Peggy and that scares you because she a memory and there's no way to really compete with that. You try not to think about it but you are worried that he will leave you in search of someone who is more like Peggy was.

Thor-

Your fear is that you won't be able to marry Thor because you don't have royal blood or even an Asgardian. It is a valid fear though, one that will even cross Thor's mind.

Bruce-
You constantly worry that Bruce is going to leave you because he's afraid that he'll hurt you. He was hesitant to even start dating you because he was afraid of Hulk'ing out on you.

Pietro-
  You know that Pietro hasn't fully gotten over his hate for Tony and so you're scared that he'll turn bad again.