What am I doing wrong?

[WipsGlitter]

Seemed to be going well, lost 10lbs although my weight fluctuated a lot. Now I appear to have put most of it back on. Feeling very deflated. Have been 5:2-ing since April.

Skip breakfast
Fruit or salad for lunch
Soup for dinner

This is the general pattern. Plus coffee!

Have you been using something like MFP to check what you're eating on NFD? If so, have you recalculated your TDEE as your weight decreased? Could it be that you're over indulging a bit on NFD?

Yes. MFP is great. My weight had gone down 3lbs since yesterday (fast day). What's TDEE?

Basically it what MFP is set to if you put it on maintenance. There's a decent explanation on the big threads (in the original posts) on here.