D&D has gone from something I felt I was good at (inasmuch as someone can be “good” at D&D) to feeling like I’m the worst player at our table. I wish our games were more about imagination and role-playing than tactics and optimization. I also wish I didn’t feel miserable and depressed all the time. That would help.
I don’t know any details about your misery and depression, but I hope this helps even a little bit: You are doing a remarkable job with your blog. You are awesome.
If you are close with your group, you could try reasoning with other players (and the DM). If not, maybe it is time to start looking for a new group. If you can muster courage, do not bottle up your feelings, let other people know. Other players may start acting like you need help with cheesing up you build, but if that is not the problem, tell that as well.
In my personal experiences I have come to understand that blunders, mistakes and low times are part of the process. But certainly there are times when you feel that you have succeeded and had fun.
When I take up gamemastering, as much as my players want to show me how fun their cheesy builds are, at this point I have stopped caring about statistical values. I do not care for digits or single ±1 modifiers. If I want, I can handwave a monster to death. What I do know is that when I spend more time building challenging combat rather than making interesting places, npcs and events, I’m usually having a bad time with pre-game preparations. But even then there’s a slim chance that after the session, players say: “Man, that was such a great combat.”