Well, first of all, and this may sound obvious, but:
Are you basing the date your period will come on the calendar day of the month? (Like, it came on August 14, so it should come on Sept. 14?)
Or are you counting the days between your periods?
The second way of counting will put you closer to the day, because months have different numbers of days.
As for regulating it--the only way you can do that is by getting on hormonal contraceptives. Other than that, you'll just have to wait till it evens out.
Tampon question: It may be uncomfortable at the beginning, but you'll get better at it. You shouldn't be able to feel a properly inserted tampon at all (this is 'cause there are very few nerve endings up inside the vagina).
You can make it more comfortable to insert by putting a bit of a condom-safe lubricant (like KY Jelly or Astroglide) on the tip of the tampon (NOT Vaseline).
Kissing girls in English at the back of the stairs, you're a honey with a following of innocent boys, they never know it because you never show it..." - Belle and Sebastian