1. I have always thought that bananas don't/didn't float.
2. I have always thought he is/was a good guy. 
3. I have decided that I want/wanted to donate my heart.

For 1 and 2, a lot of native English speakers said past tense "didn't and was" are correct.
However, all of the native English speakers said present tense "want" is the correct answer for 3.

I'm so confused.

Please tell me where you are from at the end of your answer. For example, New Jersey the USA.

Sehen Sie eine Übersetzung

I have decided is the present perfect tense (not just present tense) perfect here means completed

This links something that has happened in the past with something that has yet to happen

So in this case

I have decided (at some point in the past)

That I want to donate my heart (something yet to happen)

That’s why wanted is incorrect


I wanted to do something implies that the intention was there but something stopped it happening

But nothing has stopped this person from donating their heart … hence want

In the first two examples there is not the same link so they are using a different tense