Ok.....
Couple of things i suppose... yes, foreplay of course is essential. Make sure she's very relaxed and looking forward to it - discuss if she has any mental issues like fear or something that are making her tense up.
Is it always tight? Or just when you try to have sex?
If it's always tight, then maybe she should play with a small dildo a bit, and get in the habit of stretching herslef out a bit.

Most importantly don't panic - it's perfectly ok and just a matter of identifying what can be done to loosen her up. Worrying, of course, is the worst thing!!!

Good luck, and please let us know how things go!

CasperTG

__________________
You know, Blackadder, for me socks are like sex. Tons of it about and I never seem to get any.

My Photos:
https://imgur.com/a/80hiRdm