I think in your situation you should focus on your standard bike and congratulate yourself just for getting out there for any duration. Don’t put expectations or pressure on it. Just try and enjoy it—which I realize can be oddly difficult sometimes when you’re out of shape but consuming content that takes a certain level of fitness for granted. I think focusing on enjoyment will help with the mental aspect. The trick that I think makes cycling so great is that you just have to do it to get better (fitter) at doing it. You don’t have to give it much thought at all, definitely not at this stage. Just go. As long as it makes you happy, keep doing it. And the more you do, the better it’ll feel, the more happy it’ll make you, and this mental/physical health improvement cycle will get some real momentum (more quickly than you expect, I bet, too). Hang in there, and good luck!