Struggling to change my diet routine and cut out caffeine

I second Kate's "Small Changes Very Slowly".

There's a proverb from somewhere that suggests we 'act like an ant' to get anything done. Carry just one small stone at a time, or focus one thing at a time. However:

Matty said:

get a lot of abdominal discomfort and bloating, dizziness and at times extremely tired. 

This sounds as though you could potentially have allergies/intolerances, which could also mean being thin is a side effect of being malnourished. 

Autism tends to go part and parcel with gut-health issues, and there are a great deal of medical papers supporting this, the result of this means biological anxiety (stress), a stressed immune response, a mental fogginess and so on. I personally have found a supplement with Lion's Mane can help.

If you'd like a plan to try and work out what the causes of this are, I would highly suggest looking at the FODMAP diet as a temporary starting point. For me, this set of 'safe foods' can generally work for most, and from here, you might be able to introduce one new thing for a few days to see how you get on. 

I've mostly swapped out sugar for: honey (and honeycomb), maple syrup and date sugar. But I do know that sugars can cause problems in the gut as well including candida overgrowth. I eat a lot of fruit and a little dark chocolate. Sometimes, if you start by changing the sweet intake from refined to unrefined natural and single source (instead of a cake, berries), it's much easier to manage.

I've seen past threads with allergies and we can all be different. For instance, I'm intolerant to the entire mustard family:so no rapeseed, spinach, Brussels, asparagus. 

My son has a very unhealthy diet and really struggles to change that because he can’t motivate himself to make changes to the foods he likes. Like you he doesn’t become overweight and is very slim - if I ate what he ate I would be very very overweight! But like you there is the concern over the health effects of so much sweet food and the lack of vegetables. However he has some healthy-ish things : he likes fruit smoothies, so we buy the ones with the whole fruits crushed so there’s a bit of fibre in there. And he likes some breakfast cereals - which have added vitamins, and he likes margarita pizza which at least has tomato and cheese - relatively good things. He seems very healthy in terms of not showing any signs of problems, but I do worry about the future. Having said that I think that his life is far from easy and if he enjoys his food then I’m glad about that - even if it’s not the healthiest. He eats an astonishing amount of Jaffa Cakes - but he loves them so I don’t blame him for not wanting to give them up. I suppose small improvements are a good thing - so maybe you should focus on small changes and go very slowly if you want to change your diet. I do sympathise - it’s not easy! Good luck!

Matty said:

I looked up withdrawal symptoms online and that's scared me a little

It sounds like you may end up with anxiety causing some of these symptoms if you believe you will suffer from the same.

I found that coming off high caffeine leves was most likely to cause headaches so I just took some tablets when the symptoms presented (like you would with any headache) and this took care of them.

Keeping busy seems to help, ideally with something you enjoy so there is no need to need to find the energy to do it.

Also planning it helps with the changes for us - set your schedule to drop one can/cup every 4 days or so over a period of a few weeks and replace it with water - fizzy water with a little cordial can help but try to do this as a stepping stone to just not having anything other than water and a caffinated drink twice a day over the course of about 3 weeks.

Doing it slowly will make your body cope better with the changes and reduce symptoms so nothing to stress about.

Having a plan you can follow, check off to build confidence and see ahead will make the changes easier to cope with.

And try to eat some fruit - ideally nothing sugary like oranges or mango and keep this in moderation too.

I'm similar to you, and all I drink is Pepsi Max and eat bad food. What has worked for me before, and I am doing again now, is go from fizzy cola to sparkling flavoured water... then eventually onto flavoured water, then just plain old tap water. Although you have to be careful when withdrawing from caffeine as you'll get headaches just cutting it straight out. But I went "cold turkey" and endured about a week long headache, then it got better. Annoyingly I got back into drinking just pepsi max all the time, but now I'm trying to come off it again by drinking my favourite flavoured water and 1 can of pepsi max a day, then eventually cut it out all together.  

I don't think some people appreciate how difficult it is, not only is it our routine, it's changing a really addictive habit too. 

Don't rush into a change of your diet at the same time as cutting out the fizzy, it will be too much. But once you start feeling the benefits of drinking water and cutting out the rubbish in fizzy drinks, you'll start feeling more energy and then you can tackle your diet then. Good luck!

Tea is a necessary evil, for me, as it suppresses my appetite; trying to stick to three meals a day, with life in-between.