You are conflating the subjectivity of perception with external reality which is objective. Perception and reality are not the same thing!
Have you ever been affected by something you were unaware of? Probably. Yeah, thats proof of reality independent of perception. What if you are told by a doctor that you have terminal cancer, if reality is not objective independent of perception, then you could make the cancer go away by the power of thought, and if reality is social constructed, then if everyone agreed together that cancer doesn't exist, the cancer would disappear. The truth is however, that opinion, whether popular opinion or individual opinion, has no bearing on what is actually true, otherwise cancer could be 'socially constructed' out of existence, or the world made flat by the social construction of medieval Christians. It really doesn't take much thought to realize that social constructivism is utter nonsense.
How can you ever know that you know "the truth" by your definition?
Knowing for sure is usually very different. However there are very basic things, like the existence of objective reality itself, which can be proven merely by deduction, like I did.
The idea of what "the truth" is will always be changing
Did you know that perception/opinion of truth, is distinct and separate from the truth? Like for example, the opinion/perception of many people in the past that the world was flat being separate and distinct from the actual shape of the world. Was the world flat because that what everyone thought and then suddenly change its shape? No, you know thats absurd. So why don't you just abandon social constructivism, and abandon solipsism while you're at it?
However, you can know the truth by my definition
No thats not 'the truth', its not 'your truth', there are no personal truths, there is only the truth, and incorrect opinions which differ from the truth. Its your conceptualization or your definition or your framing of a certain thing or issue, but its not 'truth' unless its actually, you know, true.