I have a couple of thoughts. Firstly you should check over these games to see how the computer evaluates the position before you blunder. Sometimes finding it difficult to come up with a plan in a particular position is a sign that you're actually in trouble strategically already and don't realise it. Second, I think it's possible you're a little bit too risk averse. I know I was a bit like that when I was younger, always trying to play a perfectly solid strategic game with no risky tactical complications. But humans at any rate can't really play like that. Even with a positional or material advantage there is usually some point in the game where you just have to 'go for it', even if that sometimes ends badly if you miscalculate. My experience of just trying to keep things solid and not blunder was a lot like what you're describing: advantage slowly disappeared, frustration increased, blunder happened.
I can't concentrate.
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I've lost so many games after making a solid start: pieces well developed, King safe, maybe even a pawn or two up. But 15/20 minutes in I start to lose the plot. I forget what my plan is, where my opponent's pieces are, all that sort of thing. Then of course I blunder and the whole thing falls apart.
It's the same with doing tactics exercises. I start off putting on 40 or 50 points, then have a couple of failures, then panic and finish up back where I started.
I know it's not a good idea to play when hungry, tired or drunk but does anyone have any other suggestions or advice for improving mental stamina and concentration? I play 30 minute chess, can't imagine how anyone gets through a classical game lasting all day.