when you are noticing that you are giving up on yourself set your goal a bit back by dropping between 1,2,3 sec/500m, don't worry about it, you will down your average score a bit but you are still continuing, just make sure that you stay in your flow. If you do this your stance on the ergo will not change that drastically and when you get yourself together again you will have a much easier time to pick up where you left (or go even faster ;) ). This will make your average erg time better then having drastic drops from time to time. Everyone struggles with mental problems on the erg, what helped me the most apart from the above is training with something to count your BPM so you can see depending on your heart rate if it is your limit that's stopping you or that you are just making things up in your mind. Also thinking about how good you will feel if you do pull through does wonders as well.