You are clearly smart and sensitive. Some of this sounds like usual early teen anxieties and behaviors. You are just beginning to come into being the person you will be. It can be confusing and even upsetting at times. It’s normal to be preoccupied. It’s normal to be somewhat distracted.
One common explanation for difficulty with attention and memory among teens is that they aren’t getting anywhere near enough sleep. If you aren’t getting 8 hours or more sleep every night, that could account for your distractibility.
I can’t tell on the basis of a short letter whether you meet the criteria for ADHD or if there is another physical or emotional reason for your concerns.
The way to answer your question is to get an evaluation by a real professional instead of searching internet sites. Ask your mom to get you an appointment for an evaluation. If you do have ADHD, you’ll get some recommendations for how to manage it. If you don’t, you’ll get the peace of mind of knowing — and perhaps some suggestions for life style changes that could make life easier for you.
I wish you well.