Asslam O Alaikum Wa Rahmatullahi Wa Barakatuh,
You dont have to promise that you will keep fasting Monday and Thursdays. Just try your best if you can. There are several recommendation for fasting who can find in the Sunnah for example:
- It is recommended (not obligatory) to fast 6 days of Shawal.
- It is recommended to fast on the day of Arafa.
- It is recommended to fast on 9th and 10th or 10 & 11th of the Muharram.
- It is recommended to fast on Ayyam al beed (13, 14th and 15th of the Islamic month).
- It is recommended to fast on Monday and Thursday of every week because during these days our deeds are presented to allahu Subhanahu Wa Taala.
- It is recommended to fast every other day like Prophet Dawood (peace be upon him).
Now if you are married then I will advise not to take any extreme measure. Three days in a month shall be enough or at the most you can day Monday and Thursday. Your wife have rights over you and make sure you fulfill those. Fasting does kill your sexual desires, so don't over do it if you are married. If you are single you can fast every Monday and Thursday or every other day until you get married, only if you are able to do so.
There are several hadiths on this topic which I can quote. But below is one hadith which I hope will help.
It was narrated that Abu Salamah bin 'Abdur-Rahman said:
Keep a good balance in your worship of Allah Subhanahu Wa Taala and in fulfilling the rights of other people on you and stay consistent. Allah loves things which we do on regular basis and perhaps it will be easy to fast three days in a month for the rest of your life. And Allah knows the best.
Ma'aSalaam