Just over a year ago I completely over hauled my diet. I went on something called the Blood Group diet (I'm type A) and many foods that are deemed as healthy are off the list eg, tomatoes. It all linked up to what was best for your blood group.
Not only that but going gluten and dairy free was on the list too. I was quite ill at the time (suffering from PCOS... Yes there's the pill but the pill just screwed me up even more mentally so kinda ruled that out, anyways!) so I was going for a different approach with diet.
The major mindset I had for the first month was "You eat to live, not live to eat". After that things became easier. And after that all my problems cleared up (so now if I do have gluten, in particular, I'll revert to how I was over a year ago and be in a lot of pain)
I've lost 7kgs (15 lbs) since going on that diet. I've been a gym junkie for 7 years but only started to really train up hard the last 6 months. I'm doing stuff that I never thought I could do (for example I completed a 5km fun run without stopping to walk. Major goal smashed there) and training at a Crossfit type gym.
Don't beat yourself up if you slip up. And it takes time to see results.
Quite often if I'm struggling while doing exercise I'll think "If I was a zombie, then it's only really a mental mindset. Zombies can do anything. Be a zombie!" and "What would Buffy do? She would nail this. This nothing!"
It's probably kinda strange to think like a zombie but honestly, it works really well for me.
Good luck!!! 😀