Joining in the monthly club challenges here has been very motivating for me on a short-term scale - maybe there's a challenge here that appeals to you?
Over the longer term, the best motivation for me is thinking about how bad off I was when I started this 2 years ago. Shaky, brain-scrambled, weepy, no muscle worth talking about, couldn't breathe right, brownish-orange face (I was a very, very heavy smoker for 15 years). Apologies for the melodramatic movie-of-the-week tone, but when nothing else works, making myself imagine being back in that body is ALWAYS enough to at least get me out for a walk to the park.
I'm also a huge fan of pre-cooking and pre-packaging as much as I can for the week ahead, so that the cooked rice for example is there ready to go. What else... I find loads of nifty colour in my meals (red rice, purple kale, tumeric and such) rewarding and motivating as my diet growing up was pretty bland, spiceless... any colour was cheetos orange or koolaid purple.
So it's a treat to shop ahead at the grocery, which helps keep me motivated and away from a self-defeating corner store dash.
Everyone's challenges are different though. Some people find a tonne of motivation in having pictures of fit people on a posterboard (pinterest looks interesting) or they take up a hobby of collecting healthy recipies - things like that. I'm really looking forward to checking back and reading some other replies to your question, or seeing what similar threads have shown up in the past.
Good luck (and welcome to the board)!