Thanks so much for sharing this. I’ve had a very similar experience with tampons and I've worried about what that means for my ability to have vaginal sex.
Similar to what Amanda said, I've found that experimenting with inserting a finger really helped. I went extremely slowly, took a few deep breaths to calm and ground myself, and tried to relax my vaginal muscles, only going as far as was comfortable.
Another thing that helped me immensely was changing my mindset around vaginal sex. So much of the language I've heard and continue to hear surrounding vaginal sex focuses on this idea of “being penetrated” or having something “done to” the person with the vagina. Personally, this language feels invasive and somewhat aggressive (not to mention colonizing), and generally makes me tense up rather than relax. It also feels passive to me and doesn’t fully acknowledge my own agency as an active participant in that experience. So I've worked to replace that kind of imagery with the notion of my body “inviting” or “welcoming” something (or someone) inside, of my own choosing. That feels a lot more respectful and less invasive than having something being “done” to me, and it has helped me feel more relaxed when experimenting with my own fingers.