I think it comes back to the old story of if a tree falls in a forest & nothing hears it (no sentient being), is the event real. It seems unlikely there would be a forest with no sentient being to hear it, but assume there is.

So assume someone comes along later & says, "There used to be a tree standing there & now it's on the ground". We can assume what happened by the evidence, so our evidence makes it real for us, but how about the thing in itself. It seems in this hypothetical example, there was a real "experience", which we may more properly call an event, without a self to experience it. But it appears to "study" experience, we must have a self to study it

I doubt it, very much. What we know as "experience" is filtered through our unique consciousnesses, and finds a place within same based on our unique and already formed perspective of "How things are," "How I am." And, "self" is the sum of our experiences, and how we each have organized them into our gestalt of being.

No. All external reality is perceived through the senses. It's what makes it "external", the senses act as the interface. And this necessitates the presence of a subject as loci for each set of senses. And all internal experience is internal to a subject. Either one necessarily includes a subjective self.

In my opinion experience requires observation and or measurement.
This in turn requires sentience or a sentient being to perform the observation and or measurement.
A sense of self or consciousness is a prerequisite of sentience in beings of higher intelligence therefore phenomenology requires a basic frame of reference between the observed and the observer and that in reductionist terms boils down to "I am" and "that is".

I'm working my way through Kant's Critique of Pure Reason. I'm only about 100 pages in but I would say from what I've read that every experience, from the five senses or formed in concept, is subjective.

None of human pursuits are truly objective.

I think we interpret experiences subjectively, come to conclusions about them that are made up, based on mood, type of experience, and who it was with, if positive or negative. I do think we can step back, and see the event as just the facts. Ex he didn't want to.be in the relationship, instead of he had commitment issues, or my parents were not cuddly as they didn't love me or were cold.

it is helpful being in relationships to me with people who can tell me how my thinking sounds to them and be a sort of mirror of introspection, instead of me only having my own thoughts to bang around in my head.

No, how would you do that? What is experience without a self to experience?

It depends on what drugs you are using.