First off, I'm sorry that you're in such a situation. It's difficult to deal with having disabilities on top of being short on money. That aside, here's my take:

We all have needs but it's important to also realize that needs are defined by how much importance we give it and so, our level of adjustment depends on the weight with which we define a need. Example, with the vehicle. Is it possible to commute? Or would repairing it be more cost-effective? It is the little things that would help you decide, ultimately, if those other needs should come above the need for a tablet. But be it that your spouse thinks it's a good idea to buy a tablet, it is also important to talk about the reason for your partner's agreement. Yes, it could be that your spouse is trying to make you feel better. But isn't that a good reason, regardless? After all, if you're suffering from pain and depression, it is also a weight on the minds and hearts of the people around you. It might be that they want to help you but they cannot help any other way. They might see the tablet as a long-term investment. How? For one, if it makes you happy and eases your pain, it puts them at ease enough to be able to concentrate on other aspects that need to be seen to. Another important point, as you mentioned, it can help you earn money thereby earning back the money it cost to buy it (as well as earning more after recovering the cost) and potentially be an additional source of income to help with the other needs.

Evaluate the other needs, too, as I mentioned with the vehicle. Consider if there are other alternatives in dealing with the other needs. Then compare it with the tablet (you've already mentioned you're not going anywhere with the paper and pen approach). If guilt and being selfless are holding you back from evaluating effectively and logically, understand that it is difficult for your loved ones to see you struggle and perhaps seeing you happy (and productive) in the long run is a need for them, too.

Best wishes :)