I need to learn from past experiences.
It seems to me it makes sense to say "past experiences", since here experience refers an event that happened in the past. However, I do see a lot of occurrences of "past experience".
For example:
Does Past Experience Increase Empathy? (source)
Strangely, this same article also has another usage:
Similar past experiences can both help and hinder our understanding of others.
Here is another one:
How not to describe your past work experience in a job interview (source)
Best Answer
This is a tricky one. Both can be used, however preceding words may need to be edited in order for it to sound natural.
For example, in your first sentence, the simplest way would be to say:
While not sounding quite as natural, this also works:
The way to make it the most natural sounding would probably be to say:
In most cases, both will work. Adding in the my beforehand (when they are your own experiences), makes it sound even more natural, particularly when linked with the plural.
I will quickly run through what sounds the most natural for the rest of the examples you provided:
You had this one perfect, plural would also work here but Does would have to be replaced with Do, both sound equally natural
Again, perfect, but this one doesn't sound natural without a plural, so keep it as is.
This sounds more like a title to a book or article, and probably wouldn't fit in to a paragraph. However, it does sound natural in that context. This is also grammatically correct with a plural, but doesn't sound quite as natural.