Intuitively, I'd say teh-REE-sah, however, my dh and I both love Mother
Teresa and would consider that pronunciation for another dd. It's the only way we've ever heard her name pronounced (teh-RAY-sah), however, whether we'd get that pronunciation is another matter all together.
The only spelling I can think of that might be pronounced the way you prefer is Teressa, however, I wouldn't recommend that. It looks unattractive and kr8ive.
The only name other than
Teresa to get to
Tessa that I can think of is Contessa and I wouldn't recommend that either.
I think I'd just stick to
Tessa (although I adore
Tess). My dd has a friend named
Tessa Leanne (blech) -- my point being that
Tessa works.
Honestly, I don't get
Tessa from
Artemisia. Just stick with
Tessa.