It sounds like you know what you want, or don't want, to do. You just need some support?

Simple answer: If you're not comfortable then don't do it.

More complicated answer: If you feel unsure but aren't ready to say no you can ask yourself where the discomfort is coming from. Is it the whole idea or just some of it? Sometimes people are intellectually interested in something and find the idea erotic, but are hesitant to do it. The hesitancy is usually emotional. Some fear that holds you back.

In the case of a glory hole it might be around your sexuality. You could be afraid you might like it. "And then what? Will I become gay?? Will I become a cockfiend???" Inversely, you might fear you'd hate it, have a strong physical reaction and couldn't complete what your Mistress is asking of you. "Will I embarrass myself? Will I disappoint her?"

You could also be (wisely) worried about contracting an STI. You could acquire a variety of things if the interaction is unprotected. Certain types of play should involve the at-risk parties getting tested and then agreeing not to engage in high-risk play before the scene. Just know that some pathogens can't be tested for, like HPV, which is a commonly spread STI, and some people may say they've been tested and haven't, or they have but they tell you they are negative for everything...when they aren't. This is a VERY SERIOUS concern if you have a partner who is unaware of what you do and you have any risk of passing something to them. This can include kissing, where the gonorrhea you just acquired from sucking a cock or licking some balls is now in your mouth and is passed to your partner's. I will spare no words in saying that anyone who engages in risky behavior and then puts their partner at risk is a GRADE A ASSHOLE.

Anyway.

If you decide you are not interested, or at least not ready right now, tell her. If she does not respect and support your feelings then it's a red flag. It doesn't matter how cool she is or how much you like her. No one should be coerced or shamed for not wanting to do something they aren't comfortable with. In fact, it's against the law.

If you decide you'd like to try, I encourage you to do it safely. Make sure you ask your Mistress who she has selected and if she has them screened for STIs and how she's verifying this. If you do not feel 110% confident in her answer, then insist it's protected. Even if you do feel confident, I'd still tell her it must be protected.