Being even a little nervous can cause the vaginal muscles to be tighten, which can cause it to be hard for even a finger to be entered.
At this point, you may want to try masturbating and going further inside than you normally have, and see if you can stay relaxed for this to occur. Even getting a sex toy may be helpful. Now do keep in mind that if your hymn is still in tact, it may be a bit painful, and that part is normal. Plus never having gone all the way with someone can be scary and be a nervous moment too. So try to relax with yourself and see if you can get farther than your partner did. Sometimes I may take a few times before it won't be so painful. And when you do actually have intercourse, you may want to try being on top. As this allows you to control how deep and how fast he enters you. Plus it opens the pelvis up vs him being on top.
If none of this helps, then I would suggest following up with a GYN just to make sure there is no underlying cause for this to be happening.
Hope this helps, If you need anything else please feel free to ask. If you found this helpful don't forget to click the *ACCEPT* button and please take a moment to leave me *POSITIVE* feedback.