I get it, when I was your age I would take my pocket money and buy 3 chocolate bars in one day.
I think you should congratulate yourself more on what you have already achieved and not beat yourself up over the last bit. If you are not eating meat, fish, eggs, cheese or eggs or drinking milk then you have probably already reduced your impact on animal suffering and death by 95% or something like that. It will always be harder to get to 100%.
Some candy/sweets have gelatin as their main ingredient, while milk chocolate has a lot of animal product in. You could try to minimize these and look for other snacks like crisps which are more likely to be vegan or mostly vegan.
I didn't turn even vegetarian until I was already past 30. I think it's impressive that teenagers are already considering moral issues like this and making such smart and ethical decisions.
Do try and get closer to 100% over time but don't stress over it. I am in a similar situation as you, and I do (occasionally) eat sweets and biscuits without knowing if they are vegan or not.
When you are somewhere you can buy vegan stuff, buy more. Buy 5 vegan chocolate bars and put 4 in the cupboard when you get home, for example. I am not sure if that's realistic money wise for you, depends if you have an allowance or a part time job, but you may have to save money in advance.
The other thing I can suggest is to drop hints, especially around family that buy you presents for birthdays and christmas. Teenagers can be tricky to buy for. If you say in front of an uncle or cousin it's a shame I can't afford vegan chocolates/sweets hopefully they will be smart enough to remember this conversation months later when they are wondering what an earth to buy you as a present. I suppose you also need to make sure they know about the nut allergy and check on the packet or online if you do get given anything.
Also, you don't need to find products that say "vegan" on them necessarily. Some regular packets of crisps or biscuits are vegan anyway without trying. Do some research online regarding which common products found in supermarkets etc are vegan (and obviously checking re nuts as well).
Good luck!