That's irrational. You found the service valuable at the price point you paid. They're running a promotion where new customers get a credit. Another persons deal doesn't make yours less valuable... it's perception.
So fix it. You see something happening which makes you feel bad, presumably you don't want to feel bad, so find the bug in your thought processes and correct it.
How would you like to feel instead? Happy that the trade you did was fair at the time, and also happy for the people getting a good deal? Sounds good.
There's your unit test: imagine linode offer. Current result: feel bad (q: how, precisely?). Desired result: feel good.
Now all you need to do is fix it as if you were fixing a bug report - explore why and how you feel bad (observing what actually happens in your head instead of what you think ought to be happening) and what you would need to change (in your head, not in the world) so you can feel good instead. Keep trying things and rerunning the test of imagining the linode offer as a check of whether you have successfully changed anything.
The essence of mindhacking in one HN comment. You could pay a therapist loads and not get to that heart of the matter, which is the old stereotyped Buddhism trope that the world is fine and if you feel bad, it's your job to fix your feelings, not the world.
