There is a difference of opinion as to the force of the prohibition of affectionate or sensual touch, without intercourse, for a couple in your situation. One opinion is that it is a rabbinic level prohibition. Rambam, however, holds that such touch violates a Torah prohibition and incurs the penalty of lashes. Intercourse would have been a violation of an issur karet.
The prohibitions for a married woman when she is in niddah are the same as those described above. Since the temptations to which you succumbed are common in loving couples, the harchakot are designed to help couples keep control when the wife is in niddah. In addition to taking measures to avoid a recurrence (including careful adherence to the laws of yichud until you are married) and taking time to do teshuvah, you both might also find it helpful to review the laws of harchakot. You could start by looking at the articles in the "Conduct While in Niddah" section of our site.
It is wonderful that the two of you are so attracted to each other. Be'ezrat HasHem, once you are married you will be able to express your affection physically at the appropriate times. The wedding day is akin to a personal Yom Kippur, an opportunity to complete teshuvah and to make a fresh start.
May you merit to keep these mitzvot for a lifetime together and to build a bayit ne'eman beyisrael!