Praise be to Allaah.
You have to try your best to do that, and if you fall short on some days, then you have to repent to Allaah. You do not have to offer kafaarah if you did not swear an oath, but if you made that promise in words that imply an oath or vow, such as saying “By Allaah (wallaahi)”, then you have to offer kafaarat al-yameen (expiation for breaking a vow), because Allaah says in Soorat al-Maa’idah (interpretation of the meaning):
“Allaah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allaah makes clear to you His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.) that you may be grateful [al-Maa’idah 5:89]