Hi
Well perhaps I'm unusual, but I just don't have motivational slumps. And I think it's the sheer variety of what I play that keeps me interested, and has done since I was about 18. Currently I'm having classical lessons (studying ABRSM grade 8), playing in a big band every fortnight, playing in a pop/rock band, hosting a Jazz jam session once a month at my house, and attend a fortnightly pop/rock jam session. It helps being retired!
Also I've always been realistic about my ability. I always knew I'd never be a very good classical Pianist, or Jazz Pianist, or indeed a very good Pianist in any genre. But I've achieved more (I won't bore you) than I believed was possible, when as a 9 year old (in 1970) I first started playing. It took me almost 10 years to find a way into the Piano (in non-classical), and in that period I certainly had slumps, but since about 1980 It's been an important, and continuous source of pleasure in my life.
Cheers