Given the situation, seems that your mother is an ass. From what you said, she's in a pretty good spot financially, whereas this was a substantial portion of your savings, yet she sent him to YOU rather than deal with it herself. And set you up to fail. She drove the whole deal, but now just wants to stay out of it? Then why'd she push you into it? Especially if she could afford it herself.
You're getting used by both sides here. Said you have some sort of automatic repayment plan, personally if that's still ongoing, I'd just leave it alone and hope it eventually pays itself off. Pretty clear you aren't getting it any faster, nor are you getting any thanks/gratitude/compensation.
You don't lend money to friends/family, not if you have any expectations of getting it back. And you're certainly not getting any interest/benefits over it.
And not sure where you're investing your money, but there's no way you missed out on $1,000 in interest by not having $5k in your account for 3 years. That's a crazy return on investment in this environment without some serious luck (steady 6%), and savings accounts don't pay jack in interest anymore. And you got SOME of the $5k back, so can't even count the full amount. Don't make it worse by imagining some crazy amount of loss you're incurring; the loss is pretty much limited to what you gave out, plus maybe a couple hundred bucks in interest max, assuming good return...