Thanks for your help guys, got everything working again. Got all my playlists back and the information in my custom tags.
Thanks @frankz for your suggestion, I don't know why but I never thought about the possibility before of it updating everytime MB was closed because that makes sense. I actually had 2 seperate copies of my library, one in my C drive and another in my D drive (my old laptop didn't have 2 separate drives, my new one does). MB was using the one in the C drive which I assume didn't have the information the good old one in my D drive did. I couldn't figure out how to relink the library to use the D drive instead (and didn't want to because the C drive had all my playlists stored in it which was its own big ordeal to fix, but I managed to do it on my own) so I just copied the .mbl and mbl .bak files over to the C drive. This however only worked partially as all the files became unlinked from their original locations again to the old drive, which was understandable, however they did have the custom tag information in them unlike the previous files. Tried relinking and rescanning to no avail, they were still unavailable, and actually MB ended up making duplicates of every single file in my library. When managing those duplicates it was split about half and half between the old linked files and the new linked files which worked but did not have the tag info in them. I was frustrated and to be honest almost was tempted to uninstall MB and install again with a clean slate, before remembering I made a backup of my library (yay me!) so copied that whole library into the current "corrupted" library and all of the files worked again, none having the tag information (the unplayable C: linked files from the old laptop had the tag information while the correctly linked D: linked ones didn't but were playable). I ended up copying every library file from the D drive onto the C drive except the playlist files which would have been lost otherwise, due to the D drive not having the playlist files, and it was likely they would have been gone forever if I'd done this and not backed it up beforehand, which come to think of it I did. A dialog popped up when I clicked on the MusicBee icon again asking me to relink the music files (interesting to note, there was two, I'm assuming for both the library management folder and the library folder itself which I kept. The music library itself defaulted to C:\Users\(old name) to C:\Users\(new name) (the two laptops had different default usernames on them) but I had to change this to D:\, but what's interesting to note is that typing it in did not work and for whatever reason said that was an invalid file path. So I actually had to click the dots and click on the D drive that way. I left the library management folder as is since those file paths looked correct (C:\Users\(old name)\Music\MusicBee I think to C:\Users\(new name)\Music\MusicBee which was correct as I decided to leave the library management folder in the C drive). Clicked OK and everything to my surprise worked, all files are playable and those which had the custom tag information have retained them, even those which have custom tag information in the playlists are playable and have the tag information, no duplicates no nothing! And yes the playlists still work.
Thought I'd write out this detailed description if any devs would find it useful, or anybody experiencing similar problems to me might be able to benefit from this description. Thanks for all your help!