You are so boring Sheppard
None the less, I think I suffer from the following problems:
Underestimating my opponent by rating, leading to sloppy play.
Self sabotage, which is an extension of the above in that I think to myself "There is not waaaay he can win now" so then I start making really stupid decisions like stopping T3 land spam which is working well (if slowly) and start building Czar which allows him to take the advantage in the land battle, overwhelm my forces and then march into my base all before the Czar is done. That exact thing happened in one of my games.
Not being able to keep up with rebuilding destroyed mexes.
In regards to the above, forgetting that the objective is
not to rebuild mexes as quickly as possible, but to take steps to prevent them getting destroyed in the first place.
I should remember to always send troops to my expansions, because that, is without doubt, where he will attack.
A lot of small stuff (like remembering to reclaim) needs to become habit, before it can help. This will help my game to a small extent but overall strategy is what will make the most difference.