I think you are doing pretty well, but I agree, no diet coke! I don't think a few cheats will hurt, but I do think the diet is extremely important, so if you can make sure you stick to it as close as possible, that will probably help. Dark chocolate is supposedly okay ( I never ate any, but I don't like it). Try to up your protein. If you don't want to eat a lot of meat, try eggs. Also, I ate a TON of beef jerky--still meat, I know, but maybe you'd like it better than a steak.
I did use BSF, but only once per attempt, an hour before BD. I TTC for 3 months, and got my BFP on the 3rd try, and the two prior months, I had used much more BS before each attempt, and used it several times a day--so cutting back on it may have helped?
Also, definitely use preseed. I think that stuff is amazing!
All of us question our sways. It's impossible not to, since it isn't an exact science, and we don't know exactly what will work for each of us.