You may have tried to reward yourself for completing little goals you've set.
Here's a thought - why not also set yourself a penalty? Yes, set yourself a REWARD AND a PENALTY!
For example, "When I’ve exercised every day for 20 days straight, I’ll REWARD myself with a massage, manicure or good book etc."
The PENALTY could be "If I don’t exercise every day for the next 20 days, I will donate $50 to my Mother in Law!"
Or maybe ask your friend/significant other to set the penalty; they may come up with “If you don’t exercise every day for the next 20 days, you will have to wash my car every weekend for the following month!”
Have fun with it, but make it something that’s doable and find someone to hold you accountable!