I don't know how many times I had to watch this scene to figure out what this look was, and I've seen a lot of people interpret it as his mask slipping. His hand reaching for hers is him savoring the motherly love his younger self knew and held so dear.
What I'm now seeing and feeling is Aemond trying to tell her, "I see you trying, but you're too late and now I can't turn back. I've gone too far to take any of it back. Where it may have been enough, I can no longer see it. It's not your fault. We are consequences of time, and too much has passed for me to be the boy you once knew. He grew up broken and shattered, but he's put together again - just not in the way you had him before. I fixed him. A mother's love will not be enough to get him back."
There's also the muddled concept that Aemond believes he's sparing his mother from having to witness all he must do to keep to the path he chose. This is an act of love, one that Alicent will probably never recognize because he has to push her away to do it. But to spare your parent from seeing their child become something that was never intended for them will always be painful.
