Counting the syllables in that line you have 13, so to make it an iambic trimeter you have to lose one somehow. If you mark up the longs/heavies and then fit in a few shorts/lights you should see the expected pattern beginning to appear - working from the tail-end always seems to help! I assume therefore that your problem is in the first metron, dealing with the naturally long omega of 'egw' followed by the pairing of omikron & upsilon.
There is a process called synecphonesis (among other bits of jargon) where two adjacent long vowels are run together to produce one long syllable. This often happens around monosyllables, relative pronouns and 'egw', 'epei'. Taking that into consideration your line should scan according to your expected (hoped for!) plan.
Tragedy, as Sophocles and his colleagues used to write, is 'normally' quite regimental - isolate your problem points accurately and then dip into a text book (or ask here
) and Bob's yeruncle