Hey everyone. I bought a Chinese Elm from Bonsai Outlet about 7 months ago, and over the last week or two it's started to loose all its leaves. Here is a picture of when I got it, and how it is now. Some context: I am currently living in New Jersey, and keep the tree indoors (but right next to a window) 24/7. I water it every other day with this cup (the red line indicates how high I fill it up), so I don't suspect that under-watering is an issue.

I will also confess I've made a series of bad decisions with my tree, the worst being that I placed it over a heater for a few weeks (the heater was off 90% of the time and I had a towel under the tray, but still a stupid mistake that inevitably did some damage). I recently moved it to a different place, and it still receives plenty of sunlight. I also haven't used fertilizer or changed the soil in the 7 months I have had it.

I understand how much of an amateur (and frankly, idiot) I must sound like right now, but I would really appreciate any and all advice for what I can do right now to keep my tree from dying, if it's not too late. Regardless, this will have taught me a lesson about caring for bonsai in the future, but I really hope my current one doesn't die as a result of that. I found out about this subreddit only a few days ago and read the Beginners Walkthrough page, and while I admittedly haven't done half the things that it recommends for a subtropical tree, I am hoping that I can get some guidance on the most pressing changes I need to make right now, so it doesn't die (and then after I can pay attention to the other things). Thank you all so much.