an ex-gf had the same problem.
From the specialist we visited, vaginismus is a protective reflex by your v. , it's "locking up" against anything "unknown" (your partner included). It's comparable to the eye that automatically closes on an arriving object.
Vaginismus is rather psychological than physical. In our case it came from something that shocked her in the past (something that scared her bad but no harm) ; maybe something shocked you when you were a child or something else. We had to follow a therapist to find out, make her relax, understand what's wrong and then there was a long procedure by our side for her to trust "me".
Btw vaginismus happens
way more often than you can imagine. You're not like the single weirdo, the therapist told us that many woman have this problem but are too embarrassed to talk about it, thinking the problem is their fault.
First thing, don't try intercourse as long as the problem persists. Also, tell your husband that indeed many women suffer from it. He needs some explanation too so he doesn't feel like "I'm cursed, I can't have sex" because many men are cursed that way
Just in my own short-range friends' group there's already 2 girls that suffer from vaginismus (and those are only
who did tell me so we're not even sure if there's not more).
Inserting yourself is okay, but it's important that you don't "force yourself" to feel relaxed, because you and me know it's just more pressure

(the expecting pain you talk about).
Don't force, inform your husband, and see if you need therapy. There's nothing to be ashamed of (we didn't feel like it), or just see what she has to tell you about your problem.
Hope this helps.