Starting in the middle with learning, sort of.

Incoming word salad:

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There are games on musictheory.net that can help you memorize intervals on the fret board. I found this very useful about 5 years ago when I started jamming with friends and playing gigs. I've been playing for about ~13 years, and I've never been a big sheet music guy for guitar, but I'm sure you're aware of the abundance of tabs online.

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I've tried teaching lessons before, and I can tell you right now that technique is always the biggest hurdle to clear no matter how much theory you know. If you have enough technique built to be comfortable making chord shapes, alternate picking, basic stuff like hammers slides and finger placement, you should be in great shape to dive into the world of theory.

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My number one recommendation is to memorize the following three scale shapes up and down the fretboard:

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Major

Minor

Pentatonic

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It seems like everyone who's interested in theory loves to jump right into modes. Feel free to go ahead and memorize the modes if you're curious, but if you don't understand how they relate/fit into the context of the three scales listed above you will struggle to find a practical use for them. Further, I would argue that learning all of the pentatonic shapes on the fretboard is going to benefit your fretboard intuition much quicker.

Take maybe a couple sessions to memorize the major/minor scale shapes; I know memorizing a scale for the sake of memorization is boring af, so once you have a decent grasp on the pattern jump into the pentatonic scale and learn each shape on the fretboard one phase at a time. To really solidify these patterns, it helps to identify where the relative major and minor scales fit in. Also, look into how a major scale corresponds to its relative minor. Learn the chord degrees (I-ii-iii-IV-V-vi-vii°).

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Starting out, what kept me interested in guitar was learning songs written by bands I was enamored with at the time. Nowadays, when I'm playing at home by myself 99% of my time is just improvising until I find a mood/progression/lick that I can get lost in. I would recommend following a similar strategy as you dive back in. But do whatever works for you; make it fun!

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Above all, do not overthink it. I invested (it would be foolish of me to say "wasted") a bunch of time over the last decade thinking there was some mystical secret to memorizing theoretical concepts. It can certainly be as challenging as you want to make it, but if you are a genuine fan of music and have a decent ear, there is no reason that it should take you decades and decades to reach a level to which you are aspiring. Best of luck in your craft, and feel free to DM if you ever have questions or need advice!

I'm partly just tagging this to piggyback, because I'm in the same situation.

You should definitely re-learn how to read sheet music. While it's not mandatory, it can make a lot of things much easier for you. It also provides a very clean, logical way of understanding the intervals in the music you're playing.

Not sure about guitar specifically since I play piano but these are some things I think could help.

• You could learn the intervals that make up chords such as major/minor/diminshed and the 7th chords. So major being built with a major third between root third and a minor third between third fifth, minor being minor between root third and major between third fifth, diminished being two minor thirds between the notes. This could help you figure out chords on your own and voice them in different ways. Adding the 7th on top is either just another major or minor third depending on the quality of the chord.
• You could learn the relationships of chords in a key. Examples being the 5 chord of a key will always be dominant with option for a V7. 2, 3, and 6 being minor chords in a major key. 2 being half diminshed in a minor key, etc.
• For more technical options you can learn arpeggios with progressions. So you take a 1-6-2-5 progression and play 3/4/5 notes, however many you want in that chord and when you play the next chord in the progression try and play the closest note possible and play that arpeggio in the opposite direction. Somewhat of a voice leading exercise. This can help you weave melodies over chords and see the relationship and shared notes of chords(if there are any).
• Learning different voicings for chords and in different positions can really change up the sound instead of just using open string voicings and bar chords down at the bottom of the neck.
• Key signatures are good to learn if youre reading standard sheet music but also to remember what chords are played in relation to the main tonal center.