I don't know that I'm the one to try to help you with a breakthrough in your understanding of rhythm and metre. This can be very difficult and it seems to be something that students learn or "get a feel for" over time. It can, also, be a little bit subjective on the part of the poet. Sometimes stress is determined by the context, and a one-syllable word that is stressed in one sentence is not stressed in another. Also, no serious poet repeats the same rhythm every line--it becomes monotonous. So a student looks for the rhythm that is dominant in the pattern of the lines, not exclusive.
With all that said, here's an explanation that I hope will help.
When you speak in the English language, you speak in a natural rhythm. Your voice rises and falls with almost every syllable. This is inflection. You cannot, for the most part, speak sentences in English and speak at the same inflection. Your voice naturally rises and falls.
Look at the word: hippopotamus. On which syllable does your voice rise? You must get yourself to say the word naturally, like you would any other time.
When you speak the word in English, your voice naturally rises on the middle or third syllable, the -po-. That is the stress: hippopotamus. That's all there is to stressed and unstressed syllables. And any word in the English language that is two or more syllables, is marked for stressed and unstressed syllables in the dictionary. One place to start in trying to understand rhythm is to simply look up any words with two or more syllables in the dictionary and see where the stresses lie. They will be marked with a symbol before the syllable that is stressed, where the dictionary gives pronunciation.
The same is true when a person speaks a sentence. Your inflection naturally rises and falls as you speak a sentence, the same as it does when you speak a multi-syllabic word. You speak the sentence as you would at any other time, and judge when your inflection rises and when it falls.
Here's a short line from a famous Frost poem, with the stresses in bold:
Her hardest hue to hold
This line is iambic tetrameter: three metrical feet of iambs.
Here's a line from Macbeth:
Attend the true event, and put we on
If you look up attend and event in the dictionary, and learn where the stresses are, that will give you a great start in figuring out where the stresses are in the entire line.
I hope this helps. By the way, you probably figured this out already, but just to make sure you don't leave this site with a misconception, the Roses are Red poem mentioned above is not iambic pentameter, and all of the lines mentioned are not iambic. Also, a listing of letters like ABAB is used for rhyme scheme, not to indicate rhythm and metre.