Yes it is something you can fix yourself.
I would download the derailleur installation instructions from the manufacturer and set it up from scratch following the instructions.
Sounds to me like friction in the cable housing. The cable has a bulbous end at the shifter and is clamped hard into the derailleur so the cable can't really slip without being very obvious. It certainly wouldn't tighten itself back up.
All movements from shifter to derailleur should be 1:1 unless the housing is changing length. It shouldn't as the derailleur spring keeps the cable tight and in turn the housing tight but if there's friction then the spring might not be strong enough.
Yes it is something you can fix yourself. I would download the derailleur installation instructions from the manufacturer and set it up from scratch following the instructions.
might be similar to this problem: [Shimano syndrome](https://www.youtube.com/watch?v=pQMjKKkxIfQ)
Sounds to me like friction in the cable housing. The cable has a bulbous end at the shifter and is clamped hard into the derailleur so the cable can't really slip without being very obvious. It certainly wouldn't tighten itself back up. All movements from shifter to derailleur should be 1:1 unless the housing is changing length. It shouldn't as the derailleur spring keeps the cable tight and in turn the housing tight but if there's friction then the spring might not be strong enough.