Tapah means heat, producing heat, friction and heat and it’s therefore loosely translated as self-discipline. This very word actually explains the nature of self discipline. If you choose a little task and you make a vow to yourself that you're gonna do that little task everyday at the same time, this will probably generate resistance and friction in your life.
To understand that a bit more clearly imagine you make a vow to yourself to wake up everyday to meditate for an hour for example. Then your mind will make many excuses not to get up at some days. While some days it will seem to be very easy we all know that some others will not.
This very moment when you overcome the resistance and you push through then there is a moment of transformation.
I often use this very mundane example in my teacher training classes: imagine you want to cook pasta and then half way through you switch the fire off and get distracted with something else then again you remember you wanted to cook pasta and turn the fire on, then your phone rings and it's a friend you haven't spoken to for a long time. Then you switch the fire off again and start talking to your friend. Then after the call you remember about your plan to cook and again you go back to your stove and start heating the pot. This can continue like that for another few rounds but you will never get your water boiling. In the same way stopping our practice will never heat up the process for transformation which is needed to bring us to a different state of consciousness. We will always start again from zero.
One must keep the fire burning and keep the practicing continuously over a long time how patanjali states on the yoga sutra 1.14
स तु दीर्घकाल नैरन्तर्य सत्कारासेवितो दृढभूमिः ॥१४॥
sa tu dīrghakāla nairantarya satkārāsevito dṛḍhabhūmiḥ
Abhyāsa (inner practice), becomes firmly and naturally established over a long period of time when, without interruption and with constant effort, reverent and dedicated energy and great love, one fixes one’s mind on the Self, the I-AM. (translation by Shri Brahmananda Sarasvati)
So Tapah is not self discipline only for the self-discipline sake. It is self discipline for each individual to burn off the unnecessary which is hindering us to bring the real higher self forth. So practice with love, respect and continuously to allow this transformation to happen.
To finish I would like to suggest that you choose something to do everyday for a month. It can be 10 minutes of meditation every morning or chant a mantra, you could also choose a pranayama exercise and do it for 5-10 rounds, do 5 sun salutations every morning, chant 10 minutes of OM before bed or anything else that you feel that is important for you at this moment. You can also write me in case you want other suggestions and after you’ve done it, if you would like to share your experience with me, I would love to hear it.