The best thing I can offer for advice on this is, that is just a mental block you need to get passed. If you know what is causing it, then try to break out of it, if not try to find what is causing it.
But, I would like to mention. You have a right to your limits, swallowing or spitting doesn't Read more… make a bad sub, a good dom would be supportive of your limits, and not down you for them. If you are uncomfortable swallowing or having someone cum in your mouth, then that is something you do not have to do.
Just because others do, don't feel pressured into being comfortable with it.
Now, if it is something you do want to work up to getting comfortable with, the best thing to do is just take it at your own pace. Do not feel pressured into rushing the process, and just let the comfort come naturally. The pressure of wanting it, is the exact thing that will keep you from that comfort, and also ruin the enjoyment of your experience.
Key take away from this.
1. It isn't something you have to do. You deserve the comfort of feeling safe when doing activities.
2. Do not feel pressured or pressure yourself.
3. Take this at your own pace, find a partner willing to help you experiment with it slowly, and at a comfortable pace that you determine that is needed.
The biggest thing when doing these things is the big three: Consent, respect, and understanding. Never let anyone break these boundaries as you are worth more than that.