Well who hasn't done or said something they wish they hadn't?
I find things all of the time around my house that I meant to send back and should have sent back ..... but didn't for whatever reason.
My advice would be to stop anguishing over the bike.
You bought it, you kept it (am I understanding this, can you send it back...or not).
I did something similar with that yoga thing about 4 years ago. The thing came in pieces.
I snapped open the ties that held them all together.
I'm not a person who can follow directions. It was before I knew my friend who helps me things would come over. Now I'd call her....but this was BEFORE her.
The thing sat there for probably...maybe even 1 & 1/2 years...just sat in the rec room...in pieces.
Finally, another friend came over and I said, "You want it, you can have it". I never asked her what she did with all of those pipes and pieces. Never wanted to think about it after I got rid of it.
So, just move on. If you can send it back, put it in a huge box whatever you can get apart and box it up. FedEx or UPS people do that in their shops. I know one of them does because I sent a lot of appliances and things to my daughter in Denver. My friend took them to one of those and they boxed it up (wasn't cheap) but was done right. They mailed it out to daughter in Denver.
I wouldn't do that if I were you. Here's another example. I have a wreath I bought on QVC and it was so bad everyone thought it was fake (no smell).
I called QVC and they said to send it back. They sent me a return label and it went back.
Just do your best to get it into the boxes or keep it.
Most importantly, please stop worrying about it. The damage you are doing to your psyche just isn't worth it.
Anything you've gained by exercising is going out the window because of the damage you're doing by worrying so much.
Take care and please let us know what you decide. You seem like a sweet person and I think most of us here know we've "screwed" up one way or another in all of our years too.
God didn't make us perfect. He expects us to ask for help (in your case putting the thing back in the box) Ha! But whatever you decide stop feeling like you're a mess up.
We've all done things like you did. Fondly, Annabelle/Linda