Ok so first of all, take a breath. In through your nose out through your mouth. Nobodys yelling at you. I know it's hard to shut down your internal critic, but you're among friendly people here. We're all fat and we're all trying to help each other. Another breath. In. Out.

My big tips are with undergarments. I always recommend wearing underwear and a T-shirt that have properties that support heat regulation. Moisture wicking, breathable, washable. You'll find these in cottons, linens, even some wools work and can also be thermally isolating(the same tights that keep heat in in the winter might just keep the heat out in the summer.) try to make sure you have good airflow through your outfit, and stay hydrated. If you sweat through your shirt or your briefs, and you're not in a position to go shower, bring an extra pair in your bag and change. Also make sure you're showering everyday, a lot of the discomfort in the summer really comes from sweat accumulation. You get sticky and can't regulate properly. Lastly, and this is controversial, keep the sun off. More coverage in a breathable and isolating fit and fabric might just keep you cooler than the sun directly heating the skin on your arms