Your vagina - anyone's vagina - doesn't need to "get used" to any kind of entry or sexual activity. That's not how vaginas work
Any kind of entry or sexual activity doesn't cause any kind of permanent change to any of your genitals, so there's no such thing as your vagina "getting used" to it.
Your genitals
do experience some temporary changes while you're sexually aroused, and those changes are usually necessary to make entry possible, comfortable, and pleasurable. When you're aroused and relaxed, the vagina opens up in width and length. When you're no longer aroused, the vagina returns to its usual unaroused state.
From OW! to WOW! Demystifying Painful Intercourse (although it says "intercourse" in the title, this piece is also just as useful for other kinds of entry, including with fingers) will likely be very helpful to you, and help fill you in on some of the reasons people can experience pain with entry, and what to do about it.
Some questions for you:
- when your boyfriend put his finger inside you, how much time had you both spent on other things that were pleasurable to you and made you feel aroused?
- before he put a finger in you, did you feel like you
really, really wanted his finger inside you?
- did you both go really slowly?
- were you feeling relaxed and comfortable? Or were you feeling at all tense or nervous?
- whenever it started to hurt or feel at all uncomfortable, did you both apply more lube?
Whenever we feel any pain or discomfort from any kind of sex, that's a signal that something isn't quite right for our body, and it's sensible to stop doing the thing that hurts and return to doing whatever thing/s only feel really really good and pleasurable. Experiencing unintended pain in any kind of sex tends to be a turn-off for most people's bodies, so keeping going through the pain makes it more likely that what you're doing will give you less pleasure and more pain.
If we keep doing something to any part of our body when it hurts, that can make our body feel sore and painful afterwards. Like with any mild injury to any part of your body, you'll just want to be gentle with your genitals for a few days to let them heal.