If I'm a Brooklyn fan, the most useful mindset that the fanbase could have would be to prop Ben up with positivity and trust.
I am excited about the concept of what the team will look like next year with Simmons in a full training camp playing next to 2 of the the most skilled players of their position. I never viewed the trade as being made for this year, I am interested in what Simmons can bring to this team for the long haul.
But trust is a 2 way street.
For the fanbase to trust Ben you have to hope that he's locked himself in the gym and committed himself to making the next season the best of his career. He should want blow everyone away next year, not wallow in his sadness. Take charge of your life, you've gotten away from Philly now go make Brooklyn your home.
I think there are a couple of things that could help him do that.
1. For one thing, it would be nice if he chose to start dressing like an adult. Burn this outfit and fire whoever told you it was a good idea to wear it.

The ridiculous clothing aside, he does not come across someone who is comfortable in his own skin. It takes a certain level of confidence to dress like that that Simmons just does not currently possess. Turn down the flash, rededicate yourself to the craft, and leave your childhood behind.
2. Practice with your teammates. Go travel to where Durant or Kyrie is held up. Organize pickup games with the role players. Pick Nash's brain whenever you get the chance. Develop chemistry before the season so your not playing catch up when it starts.
3. Engage the community in a humble way. Sure you'll get jeers but in general pass yourself off as someone who is here to make the positive impact in the community. Make some brief appearances and then get back in the gym.
As someone who has dealt with plenty of mental health issues, IMO the best cure for poor mental health is not to be coddled but to rise up and take responsibility for yourself. No one can pull you along 24/7, you need to build yourself up for any hope of lasting change.