So, sex is never something that just "happens." If at least one person involved isn't taking intentional actions to engage in sexual activities, they aren't going to happen, just like if someone doesn't stand up and start moving their feet, they're not going to be walking.
More on that, here:
When Sex "Just Happened" (And How to Make It Happen Instead).
I don't know what you mean when you say sex: if you mean one certain kind of sex, or any number of the ways we can be sexual together. But a sexual life or relationship with someone is always something that should be able to be at a pace that works for us, both in terms of what we feel comfortable with, our own learning curve, and what is feeling good for everyone involved. If and when anyone is having any kind of feeling of fear, like what sounds like your fear you and your partner may move too fast for what you're ready for and feel comfortable with, then we've got to take that into account and make sure that our sexual interactions aren' scary for that person.
The vagina isn't really an opening that has a static size, like the mouth: it's a bit more like the throat: a closed tube except when something is inside of it, which it accommodates so long as a) the person with the vagina is very relaxed, turned on, and lubricated, and b) whatever is going inside of it is something of a size that orifice can accommodate (like a penis or a finger, when that person is, in fact, relaxed, aroused and lubricated). Here are a couple of pieces that can give you more education on that:
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Innies & Outies: The Vagina, Clitoris, Uterus and More
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With Pleasure: A View of Whole Sexual Anatomy for Every Body
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Yield for Pleasure
But it sounds to me like what really needs to go on here is for you to tell your boyfriend how you're feeling and for you two to have a chat about that, and then, when you are together, to only do sexual things at a pace that you feel comfortable with. Is that a conversation you feel you can have?
Additionally, so far, in the ways you have been sexual, has that been something where you feel like you're doing at least as much of the "driving" as your boyfriend? And where you two are communicating a lot, including when it comes to consent, with each person asking the other as they go doing things, and any person only moving forward to do something when the other says it's what they want too, and then more communicating as you go?