English question: What is the difference between “what I noticed IS that” and “what I noticed WAS that”, and ...?

Sentences from TED
Article: The brain-changing benefits of exercise
Tedster: Wendy Suzuki
...
I went to kickbox, dance, yoga, step class, and at first it was really hard.
But what I noticed is that after every sweat-inducing workout that I tried, I had this great mood boost and this great energy boost.
And that's what kept me going back to the gym.
Well, I started feeling stronger.
…
On the above context, most sentences use past tense except “what I noticed IS” and “And that IS what kept me…”, my question is, if I change the “what I noticed IS” to “what I noticed WAS”, and change “And that’S what kept me …” to “And that WAS what kept me… “ , what will be the difference – I mean how to make a choice between present tense and past tense while all the things happened in the past.
And I know “is” = present tense, “was” = past tense. In the context, “is” happened in the past, and I kind of know that when “is” is a long-lasting fact, then we could use “it’s”, but I also found, sometimes “was” was used to refer to the “I feel it’s also a long-lasting fact”.
Did I say it right? Or I totally miss the point or what I say is gibberish? For example, in the context, instead of using “is”, if I change “is” to “was”, what is the change in meaning? Please help.
Thanks, all. I am an English learner.
You help will be much appreciated.