At 18 years old, you say that “recently (3 or 4 years)” you’ve been struggling. Do the math. Three or four years is 16 – 25% of your life! That’s a cause for concern. Since you haven’t been professionally diagnosed since you were three years old, I urge you to stop trying to do so on our own.
It’s long past time for an evaluation. Look for someone who has some expertise in late diagnosis of autism spectrum disorder (ASD). I can’t make a diagnosis on the basis of a letter, of course. But what you say suggests ASD should be considered. It’s not at all unusual for people who are on the high functioning end of the spectrum to be misdiagnosed or to go without a diagnosis for years.
Regardless of whether you are on the autism spectrum, there is something amiss. A licensed clinician will determine a diagnosis, help you understand yourself and present you with a treatment plan. You can then discuss how you want to go forward in your own healing.
At 18, you have a long life ahead of you. Getting treatment now for whatever is distressing you will make it likely that you can have the productive, calm adult life you desire.
I wish you well.