So my problem is, when I tune my guitar, it will be perfectly in tune, but when i decide to play higher than around the 7th fret, all the notes are almost a half step flat. This happens on all my strings. When I play on my high e, the 12th fret octave e is an e flat. It gets ever worse on the higher frets (around 17-19).

Also, the 5th, 7th, and 12th fret harmonics cannot be played unless I put my finger almost another fret forward. (Like the 5th fret harmonic is almost on the 6th fret, so on and so forth) This only happens on my high e, b, and g strings.

What is wrong with my guitar, and what do I need to do to fix it? I'd preferably not want to take it to the shop because I'm really low on money. Any help is appreciated. Thanks

PS: The guitar is an Epiphone Les Paul Custom