Experientially I have discovered this to be true but I don't think I could explain it in a good way.
Interestingly I am not sure that I would agree that "absence of pain = pleasure" can be "discovered to be true experientally" -- at least not fully.
Everything we are doing here in this discussion is defining terms and attempting to attach words to feelings. The only way to be confident that "Absence of pain" equals "pleasure" is to assign in your mind the meanings of these terms and then hold them firmly. Cicero's objection that "absence of pain is not equal to pleasure" is a perfectly reasonable assertion to many people, and it isn't met fully by saying "your definition is erroneous." Who gets to set what the "right" definition is?
That's why I think this statement is hugely important: "The fact that the name of pleasure was not customarily applied to the normal or static state did not alter the fact that the name ought to be applied to it; nor that reason justified the application; nor that human beings would be the happier for so reasoning and believing."
The "ought" in that sentence then has to be explained, and it's going to ultimately be a matter of your ultimate views of the universe. If life is a privilege and a short-term gift to be treasured, then we will see it as a pleasure. If life is a prison and a burden and a torture by the gods, then we'll see life as a pain.
I suppose yes you can introspect and learn to see that life IS really a pleasure, but in the end I think you end up needing to add the philosophical viewpoint to reach the ultimate understanding. As Lucretius says (paraphrasing) it's not the light of day that opens our eyes to these things, but a scheme of philosophic contemplation.
Also:
PD12. A man cannot dispel his fear about the most important matters if he does not know what is the nature of the universe, but suspects the truth of some mythical story. So that, without natural science, it is not possible to attain our pleasures unalloyed.
PD19. Infinite time contains no greater pleasure than limited time, if one measures, by reason, the limits of pleasure.
PD20. The flesh perceives the limits of pleasure as unlimited, and unlimited time is required to supply it. But the mind, having attained a reasoned understanding of the ultimate good of the flesh and its limits, and having dissipated the fears concerning the time to come, supplies us with the complete life, and we have no further need of infinite time; but neither does the mind shun pleasure, nor, when circumstances begin to bring about the departure from life, does it approach its end as though it fell short, in any way, of the best life.
PD21. He who has learned the limits of life knows that that which removes the pain due to want, and makes the whole of life complete, is easy to obtain, so that there is no need of actions which involve competition.