It sounds like you do not have a very strong foundation in rhythm. This may be a hazard of being self-taught. There is a literal/mathematical understanding of rhythm that is pretty simple. One whole note lasts as long as 2 half notes, or 4 quarter notes, or 8 eighth notes, or 16 sixteenth notes. Each time you go down a note type, the note is twice as fast. A dotted note is 50% longer, so while a quarter note lasts as long as 2 eighth notes, a dotted quarter note lasts as long as 3 eighth notes. A triplet is kind of the opposite of the dot, fitting 3 notes into the space of 2. So 3 triplet eighth notes lasts as long as a quarter note.
I think you should spend some time with the metronome just counting out loud and clapping or drumming with your hands. Set the metronome maybe around 60-70 bpm. That will be a quarter note in a fairly slow piece. Just count with the metronome: 1,2,3,4 1,2,3,4. Those would be the beats of each measure in 4/4 time. Now start with a couple whole notes: clap or drum when you say 1 each measure. Now half notes: clap or drum on 1 and 3. Now quarter notes: clap on 1,2,3,4. Now when you go to eighth notes, you need to *subdivide* each beat into 2 equal parts. A lot of people count this by saying "and" on the second eighth note. So you would count "One-and-two-and-three-and-four-and" and clap/drum with each of those. But you can say or think whatever works to understand and execute 2 evenly spaced claps for every click of the metronome. Now pick a pattern that uses quarter notes and eighth notes and do that. For example, quarter note on count 1, two eighth notes on 2, two eighth notes on 3, quarter note on 4. "One two-and three-and four". Make up a few patterns like that and try them. Another thing to practice would be eighth note rest followed by eighth note. Count "one-and two-and three-and four-and" but rest on the clicks and only clap on the ands. When you get bored with eighth notes, try splitting each click into 3 equal subdivisions (triplet eighth notes) and then try four (sixteenth notes). Then make up some more patterns, or try to count and clap a melody from your sheet music.