1. i had a normal echo in august of 2020. am i still in the clear for not having an anyeursm, or can they form in three years? 2. is it normal to feel an artery right under the breastbone? i feel a pulse and possibly a vein? i'm thin?

It looks like you have anxiety rather than a pathology. The thinness makes it easy to feel the pulsations and most people you could actually see the pulsations in the neck when they are thin. If you do not have a family history stop worrying and enjoy your life. I am not sure what type of echo you had however make sure to keep in contact with your primary provider.