I suggest that you talk to a *different priest *and get his opinion. The priest who has already given you his opinion may know more factors surrounding your promise than I do. As ONE example only - if one has made a private vow to God but did not fully realise what they were doing at that time, then the vow would not be valid and no dispensation would be required - but in the common course, if one has made an invalid private vow, the priest should explain why a dispensation is not necessary.
A private vow or promise to God can be sinful - to what degree depends on surrounding factors. It is absolutely impossible for me to state whether the matter is sinful or not since I do not know all the factors involved, nor am I entitled to know.
Canon Law tells us (as already quoted in previous post) that a private vow or promise requires dispensation from a priest. Were there no sin attached to breaking a private vow or promise, then I would think that a dispensation would never be necessary. But clearly Canon Law does state (as already quoted in previous post) that a private vow or promise does require dispensation, therefore Father should have dispensed you, I would have thought.
I am NOT a canon lawyer nor even anywhere close - I am giving you merely my private opinion.
Promises and vows **
2101 In many circumstances, the Christian is called to make promises to God. Baptism and Confirmation, Matrimony and Holy Orders always entail promises. Out of personal devotion, the Christian may also promise to God this action, that prayer, this alms-giving, that pilgrimage, and so forth. **Fidelity to promises made to God is a sign of the respect owed to the divine majesty and of love for a faithful God. **
2102 "A vow is a deliberate and free promise made to God concerning a possible and better good which must be fulfilled by reason of the virtue of religion,"21 A vow is an act of devotion in which the Christian dedicates himself to God or promises him some good work. **By fulfilling his vows he renders to God what has been promised and consecrated to Him. **The Acts of the Apostles shows us St. Paul concerned to fulfill the vows he had made.22
2103 The Church recognizes an exemplary value in the vows to practice the evangelical counsels:23
Mother Church rejoices that she has within herself many men and women who pursue the Savior’s self-emptying more closely and show it forth more clearly, by undertaking poverty with the freedom of the children of God, and renouncing their own will: they submit themselves to man for the sake of God, thus going beyond what is of precept in the matter of perfection, so as to conform themselves more fully to the obedient Christ.24
**The Church can, in certain cases and for proportionate reasons, dispense from vows and promises25 **