Bliss is Ignorance

One of the problems I have when I play is that I am usually pessimistic about my position. I have no trouble spotting my own positional weaknesses, and the good moves for my opponents, while any weakness I spot in my opponents position can be usually defended. So I spend most of the game thinking I am losing. However, when I put the game through Fritz etc it usually turns out that my position was far better than I thought, and my "losing" position is often slightly plus for me.
Over the last two weeks I played 2 games where I thought I had begun to kick the habit. In both games I felt the position was OK and that I was in fact doing well. That was until Fritz put me straight.

The diagrammed position was reached after my opponent had earlier sacrificed a knight for 2 pawns, and chances of a kingside attack. I avoided the worst of it and just assumed I was playing the effortless, defend-swap, defend-swap game where my extra piece would be enough. So I played the obvious
27 ... Rh4 Now this turns out to be a horrible move, for very simple tactical reasons. 28.Rxf6! exploits the position of the rook and knight and leaves White ahead after 28 ... Nxf6 29.Bg5 Rh5 30.Bxf6+ Kg8 31.Be7 Fortunately for me my opponent was probably thinking along the same lines as myself and allowed the game to finish "normally" by
28.Bd7 Rxh6 29.Bxh6 Bg5 30.Bxg5 Nxg5 31.h4 Nh7 32.Kf2 Nf6 33.Bb5 Nxb5 34.axb5 Ra5 35.Kf3 Rxb5 36.Re2 Rb3+ 37.Kf2 Ng4+ 38.Kg1 Rd3 0-1

In my second game a similar thing happened. I had spent most of the game defending, but was pleased after my opponent chose a line where I thought I had everything covered.
I had just grabbed the pawn on a4, in part because I had seen that Rf7 shouldn't cause me any troubles, or so I thought.
30.Rf7 Rxf7 31.Qxf7 As both players were under 5 minutes for the game I played my next straight away, and in doing so should have lost the game. To be honest I didn't consider any other reply, which meant I missed the far stronger 31 ... Qh6! And so the game continued "normally"
31 ... Qf8 32.Qc7 Qf6 33.Rf5 Qg7 34.Rf7 Qh6 35.Qxe5+ Kg8 36.Rf4 Re8 37.Rg4+ At this point my opponent offered me a draw, which I gratefully accepted. ½-½

Now in both games I thought I was doing OK and 1.5/2 would be a score that reflected that. But clearly this wasn't the case. So what is the lesson (at least for me)? That scores tend to match what I "believe" the result will be, not what the result should be? Maybe. And if so I might be better off actually knowing less about chess, rather than more!