It's a crap-shoot whether otc antiemetics really do anything to help. On the other hand, the script ones only do little, if anything, to help. They *may* help keep the SN down a little longer. You are going to vomit whether you take an AE, or not. Having a 2nd glass prepared for when you do vomit is the best advice.
You'll have to figure out how to deal with your SI on your own. Being sure you want to ctb is a good first step. If you have doubts, or hope that things might still get better, you'll probably have a lot of SI kicking in at the moment of truth.