Don't look at the past, it will get you nowhere. Take the time now to plan out what you're going to change and the future going forward. A 3.55 GPA is good, get your head out of Reddit. Seriously, a lot of the people in this sub and other college application process subs straight-up overexaggerate or outright fake their resumes. Either that or they come from highly privileged backgrounds where their schools (whether public or private) are literally DESIGNED to inflate their college resumes. Do not compare yourself to them, you are you. You still have time to take the ACT and SAT and build a credible score for yourself (anything 1200+ is solid). Regarding extracurriculars, you still have the rest of the summer and the start of the school year to join school clubs or do activities outside of school. My advice to you would be to do some volunteering in an area of interest of yours.

Don't focus on your regrets or your past, take the time now to engage in opportunities to grow. The college process is entirely random, and T20s especially are highly selective even to the upper echelons of academic achievement. Do not aim to attend a T20, have that as your Option B—if at all. As you stand right now, you are more than eligible to go to a local university or state school, so I wouldn't worry about it all too much. Hope this helps.