It sounds like you'd feel more comfortable waiting on that coupon for a few more days. It also sounds like you've got a good bit of shame wrapped up in your financial situation, which I think is normal and very valid. (I struggle with feeling bad about money myself, not making enough of it, not being able to be independent, ect.) So in my personal opinion I think it would help your mental health more in the end to wait and see if you can stretch the dollars you have left till your next disability check.

When that check comes in it might also help to sit down with your mom and let her know what you can and can't pay for. Just really try to have a constructive discussion on what you both can change and what you can't as far as budgeting goes. But I know that's easier said than done, and your situation also might not allow for that candid of a talk. Just throwing that out there though.

All my love and support go to you, my dear. I've been in that dark place between checks most my adult life.