Hmmmm, there are several theory sites on the web which might help you to bring back all the forgotten stuff:
https://www.dolmetsch.com/https://cnx.rice.edu/content/col10208/latest/https://www.musictheory.net/https://www.teoria.com/https://www.emusictheory.com/Have a look at them, it may help.
Your other problem is a bit more complicated though. First, I have to agree with the previous posters: it is no good idea to start new pieces after such a long time without a teacher, it may give you some fun for the next 3 months, but it may as well give you trouble for the next 3 YEARS.
However, of course we can understand how tempting a piano can be... My suggestion: 1. Read this forum. It is a perfect guide line to piano technique, practice methods, etc. You may have noticed the "Index of the Piano-Forum" at the "PF-Site" -board. Take also a look at the "Submit a Link"- thread in "Anything but Piano". You will find some very interesting threads, and it will cost you weeks to read through all of them (although it is VERY benificial). As a start I recommend you look at this list by Bernhard:
https://www.pianoforum.net/smf/index.php?PHPSESSID=b235195451d671dfff4841ef1de363e1&topic=5767.msg56133#msg56133and the free and most discussed Piano Practice book by Chang:
https://members.aol.com/chang8828/contents.htmNow, these will not substitute a teacher! They can be an inspiration, a guideline, a help, but they cannot replace the work of a good teacher (see various threads about selfthaughts, etc.). You seem to have understood this, and therefore I suggest that you take your old pieces you once played and very slowly, without any haste, with full control, concentration and self-dicipline, you try to relearn them. Start with the very easy ones, look at your fingers, try to apply what you read here, but again! If you are not sure about it, stay away from the piano and wait till you have your teacher around. On the other hand, if you only do very small practice chunks (15 min) and not too many of them, and if you pay attention to your body and your movements, if you do not exaggerate it, if you post your qestions and problems you are facing, here on the board, you could actually attempt to get a feeling for the piano.
Good luck