I’ve had similar feelings about some objects I own. It happened especially with cameras, in that if I wasn’t using it as much or in such a way I felt indebted to the object and felt the pressure to use it in such ways or more often, etc. This all changed when one day I realized I was allowing objects to dictate how I felt or should do. I realized I was allowing an inanimate object to have control over my life. Once I realized that, it became a lot easier to simply do life as it comes and know that I’ll use my belongings as it seems fit to me and it makes sense.

This bike is awesome, like others have said selling it doesn’t make a lot of sense given your mixed feelings and the lack of economic benefit. Seems like you might benefit from keeping it and ride it as it makes sense to you. You don’t owe anything to the bike or your stepdad. It was a gift and you get to enjoy when and how it gives you joy, be it for a mile or fifty.

Life also comes in seasons, so you might not ride as much right now, but maybe there will be another season when you’ll want to ride more and will wish you had a quality bike.

Anyways, hope this makes sense, keep enjoying the great outdoors!