You cannot develop internal motivation. In my experience, when you have some vision that you are excited about, motivation is not a problem. That vision can take many forms, but it's often some concrete outcome that you understand how to accomplish, and desire to achieve (for whatever reason).
I completely agree on vision point. every indivisual need to have clear vision on what you want to be in coming 1 month/year or couple of years. Good to have both short term and long term goals. achieving shortterm goals motivate for longterm goals. I give more importance to personal than professional.
I feel motivated when I look at my goals. Definitely I do sometimes feel down start thinking it is impossible to achieve. For such days I prepared list of things I achieved so far and I am grateful for. It can be as simple as the day I got independent and started earning, Bought new jewellery to my mom etc. These days are very special and bring lot of joy and inspiration to perform better and work toward my future goals.
Also never compare my journey with other people. everyone's journey is different. Having few common things doesn't mean you both are same with same level of capacities. each person's tolerance, will power, IQ different. This understanding made me to not get demotivated by looking at some peer achievements.