Tightening of the vaginal opening is one of the effects women can experience from the loss of estrogen. The type of lubricants that offer the most "slipperiness" and the least resistance is silicone; Pink may be the most popular at MiddlesexMD in that category.
It is possible to gradually, gently, and comfortably stretch the vaginal opening by using vaginal dilators. These are available in a set of graduated sizes; start with the smallest (and plenty of lubricant) in daily exercises and, when comfortable, progress to the next-larger size.
Only rarely is surgical modification appropriate for addressing this condition. With patience, women can typically achieve comfort with dilators and lubricants.