Are there any steps I can take to prevent mild inability to tolerate vaginal penetration from getting worse?

Yes. First you need to be in stable long term relationship. Second u need to make sure there is no underlying pain syndrome this is arising from. Use lubricants, foreplay, positions where u are in control of penetration until things become more comfortable.
Vaginal Penitration. You can get vaginal dilators on line and insert them at your own time till you make sure you can tolerate the largest one. However, it may be best to see your gyn and make sure there are not other problems going on.