Since vaginismus has already been ruled out, difficulty with penetration can still come from other physical causes or from anxiety around intercourse itself. Both are treatable, but they need a proper evaluation to pinpoint which one is affecting you.
Next Steps
A pelvic floor physiotherapist and a sexual medicine specialist together can assess this from both angles. It's worth raising this with your fertility specialist too, since assisted conception options exist if penetration continues to be difficult.
Health Tips
Please don't see this as a permanent block to conceiving. There are several paths forward once the cause is properly assessed.