How do you know that there is a communication going on and in what absolute sense does it occur, without resorting to any certain sense as certainty always comes up short, never converging to actual truth and is, therefore, not suitable for matters of truth. Any meaning given to any instance of language depends on subjective based grammars that give life to words, indeed conforming to a regime of, ultimately synthesized, grammars which don't exist without a subject of self-governing origination. Every measure of consistency requires a necessary foundation, with the foundation always being some imagined standard based on certainty, that commonly found tool of the mind that never really tells you anything. The emprical, material, and even biology and its evolutionary theories? Knowledge of their objects always depends on interpretation and interpretation is always not objective but instead subjective. And for there to be a commitment, there must be a personality. But personality is, logically, dependent on a regime of truth and it is codified by the user of this regime—a mess of endless self-reference, as truth is wont to do. Really, at this rate, I'll eventually converge to the truth that there is no truth, that real truth is not, instead of being is. Indeed, I'm going one truth further.

Perhaps it comes down to how we define truth. Truth, for me, is not some sort of ontological object as the word "exists" may imply, and it requires nothing absolute.

Truth is an artifact of communication as experienced by those who are communicating. It's the semantic non-randomness in language. We share expectations regarding the consistency between statements and the degree to which language predicts empirical observations.

If none of those expectations are met, i.e when all truth is gone, then language loses its function.

Indeed, since truth does not exist, language and communication is impossible. But the truth predicate for propositions isn't necessary, as truth doesn't exist to legitimately create a legitimate need for any sort of necessity. So, instead of searching for the truth predicate, and as grammar and syntax, those subjective and arbitrary creators of languages and communications, are the ultimate sources of meaning rather than any inherent meaning itself, why not look for something else? Or establish something else? Like untruth and uncertainty?

I think it's a mistake to assume that two people are communicating with each other just because they're saying the same words. Some of the worst things that have ever happened to me have been the result of people thinking they are in agreement despite having totally different ideas behind those words in their heads, and in retrospect I would have rather had arguments.

I'm sorry to hear about these bad outcomes from misunderstandings. One useful tool I picked up from Nonviolent Communication (NVC) that might help you is to request the other person repeat back in their own words what they heard you say/request. It definitely feels clunky because we don't typically ask this of people. Proactively as the listener you can offer to summarize the thoughts or request of the speaker to make sure you really received what they were saying, and ask if there's any part of it they want to clarify. "Let me make sure I understood you. What I heard you say was..." Maybe a useful analogy is to think of it as the md5sum of the exchange. I don't tend to find arguments clarifying because true listening breaks down (further).