I have started listen to knowledge recently and I, suddenly, realized that I have been insincere...

in the past. ( Did basic course 10 to 12 years back)

This is bothering me too much. I want to progress on the path, please guide me on how to overcome this feeling.

Spiritual path demands two things - intensity and authenticity. There is no point brooding over the past. At least now, you have realized, this very thought you want to progress will give you the right direction. Intensify your sadhana, have one pointed focus, listen to knowledge , satsang and don’t ever misss an opportunity to do seva.

If a room is dark of 100s of years and when the door is slightly cracked open, it illuminates the entire room. The light doesn’t need 100s of years to spread brightness in the room. Base your life on commitment rather than feelings. Feelings come and go. For the next 15 days , if you do good seva, sadhana what if you feel good about yourself? Then you might stop doing sadhana feeling everythings fine. Todays’s feelings will not be there tomorrow. But a strong commitment will remain. Initially you might feel a little challenging. Commitment is that which stretches your capabilities. The more you come out of your comfort zone, the more confident, happy and comfortable you will be.