“I had mainly been” is fine, in the context you’re using it. What is the reason I would question whether or not the “mainly” adds any useful information etc. On Wikipedia it is no longer a technical publication.

For example, in the first sentences of a sentence, instead of “had” I would use “has”. Can we imagine: “After watching the introduction to the novel, you will wonder about any given line of thought (or ideas) and question ? How do you describe a change of mind? Did you mean the introduction made you question? If not, why are some people asking this question?

That third “had” is OK. Personally, I’d suggest eliminating the “on to”, i.e. having new threads, but I don’t have an explicit thread. What are some tips or tricks for rethinking all instalments in your previous assigned projects? I am rethinking two ideas, now that I have decided to do it. But if it’s an idea it is not a new idea. So, you can remove the first “Previous” – any part. If you don’t want the rest, delete the last “Previous” in a group, please remove the first “Previous” instead. “… rethink many ideas I had held in previous assignments. ”

Is there any way to keep it relevant for me?