I second the FontForge suggestion, but...

I've actually already made my own font in Photoshop, and I don't need to do any further editing at all;
all I need to do is to import my letters and save them all as a useable font file.

A "useable font file" consists of glyph outlines, which you'll have to draw as splines, tables of metrics and kerning, rules to assemble accented vowels and the like from reused pieces, and optional features like ligatures and sets of alternate glyphs.

Whatever you drew in Photoshop is an image, and as such it can be no more than a reference to redraw character shapes and measure metrics. Unfortunately there is no "importing letters", at best automatic attempts to trace bitmaps to obtain a first draft of the glyphs.