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.
Thanks, all. I am an English learner.
You help will be much appreciated.