I don't mean to sound flippant regarding your opinion of Aquinas' Summa Theologica.

It's a five volume work. It deals with Catholic Dogma.

You profess no faith or belief.

I'm Catholic. For no other reason I appreciate Aquinas' intellectual approach to this topic.

Why is the burden of proof on me to prove anything?

I'm stating what I believe. I'm Roman Catholic, or more broadly I am Christian.
Both philosophically and spiritually.

I'm confused why I need to prove anything exactly?

or what I'm trying to prove?

As I say logic dictates that all biological life has a beginning and an end. Birth and Death.
Logic dictates that unless something can be "proved" (whatever that means) than it doesn't exist. Therefore logically again, there is no after-death...it's nothingness.

So as I say this is where your logic leads you...to nothingness...so all the stuff that comprises your life, the stuff you've read the people who love you and everything in it.

Eventually ends in nothing.

that's where Logic will take you.