For me exercise is like good food.  When I fit it into my day, I feel as if I have done something good for
both my mental and physical energy, as little as 20 minutes gives me a boost and more confidence to
face the day with clearer more positive thoughts and energy
 
It also creates a structure to my day....so if you feel you do not do enough exercise,
just add a 20 minute slot of something extra into your day:
a faster/extra walk, a yoga session, a cycle rather than taking the car,
a Joe Wicks session on line at home or a gym session (more of which are now on line so no fees
).  
I do fall off the exercise wagon too, but each time I get back on ......
it is a MASSIVE wellbeing boost  - Keep trying it absolutely helps to keep you well and is something from you to you