Sorry to say it, but IMHO your psychiatrist is an idiot. Lithium levels should be checked regularly, regardless the dose. The reason isn't only for toxicity, but also to see if you're in the therapeutic range (which is pretty narrow).

In all seriousness, your situation is pretty fuked up. Who gives a flying flip if your mother doesn't think you should see another doc? You're 23 years old. You're an adult. If you're as sick as you seem and your parents aren't able to take care of you, the government will. Get on a computer. Apply for social security benefits - that's what they're there for. Your entire life needs a reboot.

ETA: Sorry for being so blunt - I'm not trying to criticize and of course I'm not walking in your shoes - but better to put it straight than to have someone commit suicide because of this illness.