Is it normal for testosterone to cause significantly increased mood (elevated well-being, and feelings of mild euphoria)?

Yes!: If you're taking supplemental testosterone, the answer would be "yes." but you should be cautioned (no, make that a severe warning), that as (i am assuming), an otherwise healthy 18 yo with a normal baseline "t" level, by supplementing with any androgen at a critical time in your growth and body development may be doing yourself harm, and may become "dependent" (to maintain your "t"). Don't do it!