Actually, sitting down is a lot easier than standing up. Just keep practicing. BTW, I don't know if you know who Beaver Felton is, but he owns Bass Central in Orlando, FL, and he's been paralyzed from the chest down for over 20 years, and a better chops oriented bassist you will never see. I would suggest dropping him an email and telling him about your situation, and I'm sure he'll have some inspirational words for you.
www.basscentral.com.