A few thoughts:
1. Have you tried practicing EFT? Emotional Freedom Technique, also known as “tapping”. Search YouTube for “EFT”+[what you’re describing]. You might feel silly doing it at first, but it works for a lot of people. It’s helped me greatly.
2. Another approach is exercise. Studies have shown that aerobic exercise (any type you enjoy whether solitary or group/team) can help with what you’re describing. People with ADHD specifically can benefit from a mix of aerobic exercise and activities that are challenging, requiring skill and/or coordination. (Martial arts, dance, golf, etc.)
3. Discuss your meds with your doctor. Sometimes negative thoughts can be a side effect and so another medication might be a better fit.
4. This phrase is particularly concerning: “Friends and family say I’m a nice person but I know that I’m not really”. You have a different perception of yourself than your friends do and it would be helpful for you to address that. Have you discussed this with your therapist? You care for your nan and volunteer at an animal rescue. People who aren’t nice don’t do that. There’s something inside you that’s affecting how you see yourself.
5. Know that this isn’t you, this is your brain. Your brain is essentially playing tricks on you. Be gentle with yourself. Do some research on “Limbic ADD”. It might help you.
I hope that some or all of this helps. Kudos for asking for help.
Wishing you the best.