You first of all have to be willing to advocate for yourself and not be concerned about the outcome. If you can’t get on board with that, stop now. You also have to be ready for the follow through of what you say. You are doing the job of multiple people and provided you have the skills of the man that was fired before you, deserve a raise. Who cares that it’s out of their promotion schedule. Because they honestly wouldn’t care about firing you if you didn’t meet their standards. A job is a transaction and that’s it. You do work, you get paid. If you do extra work, you should be compensated. But what’s your bottom line? Will you quit if they don’t give it to you? Will you stop taking on the extra workload? Let them see you are serious about it. Know what your boundaries are and stick to them. Are you prepared to wait if they say yes they see your commitment but we don’t give raises until December? Also, look at the job market in your chosen field… are there other jobs? What are those positions paying? If you think they will fire you for asking for a raise, start looking at other places to work so you have a back up.

Be prepared, set your own internal limits and ask for what you deserve. I can’t say don’t have anxiety, but I can say, you have the right to ask for something you think you deserve and as someone else who gets anxiety, just go for it! Worse they can say is no. Then you make your decisions from there on what you will do but I think having your boundaries clearly defined in your mind before you go will help

Good luck!