For 15 years, I used my business name; another company trademarked the name in 2011. Am I toast?

You are not toast, not yet. However, you may want to take action soon.

Could I be infringing a BOOSTERS trademark?

You should check with your attorney and your insurance policy to better understand your rights and responsibilities.

Can I add the ™ symbol to my website ?

Definitely Maybe. Trademarks do not protect websites directly. Trademarks protect consumers.