A thousand times "YES!" to pretty much everything you said in your OP.
I'm three years in to therapy and my defences/resistance have been strong throughout, which has meant lots of frustration and self-criticism on my part.
There have been signs of some significant shifts fairly recently but the defences still show up massively at times - minimisation, denial, avoidance, dissociation etc - and it often feels like for every small step forward there are a few giant leaps backwards.
I'm not sure whether it's true that you're not engaging. For me, "engaging" isn't just being able to easily form and clearly articulate your thoughts and feelings and answer T's questions clearly, fully and accurately.
On too many occasions to count I have argued to my therapist that nothing was "that bad" and that she's being over-dramatic and that I can't possibly have PTSD because nothing traumatic has ever happened to me. I have shut down, I have been unable to speak (even though at times I really wanted to and had the words - I just couldn't get them out), I have stared at the wall, I have felt excruciatingly awkward and wanted to peel my skin off and step out of my body, I have stared at her perplexed as her words just feel jumbled in my head and I can't quite shift the fog to work out what she's talking about because my head is just drifting or full of white noise...
On none of those occasions were T and I having a fluid, articulate conversation about my issues, thoughts or feelings. But a whole lot of stuff was going on for me, even when I was just sitting in silence staring at the same spot on the door frame for 15 mins as a rising panic, which I didn't understand, seemed to take over my body.
So, I was engaging on some level. Just not in a way that I thought felt very productive or that felt like was the "right" kind of engagement.
For what it's worth, I don't think you're failing at therapy. Therapy is hard. And progress isn't necessarily linear. And it's a very individual process.
We're showing up. We're doing our best. We're finding our way through...