With patterns like this I found a couple of things helped me to work out what’s happening.
The first was I stopped focusing on beginning and end of rows and instead thought about the centre of the sleeve, this is the part which has to stay the same regardless of what happens each end of the row. One way to keep the centre panel in pattern throughout the sleeve is to place a marker each end, you can do this right away if the sleeve has a selvedge stitch before the diamond pattern begins,
Selvedge, marker, diamond repeat, marker, selvedge.
If the sleeve does not have a selvedge then wait until the first increased stitch and then place the markers
Inc st, marker, diamonds, marker, increase st
If you always increase outside the markers the diamonds will remain the same throughout the sleeve. When you work each row you would work stockinette to marker, slip marker, work diamond pattern as written, slip marker, work stockinette to end of row.
You can do this for the whole sleeve, you won’t need to work out how to increase the diamonds but neither will you have to give up on them. It will still look very neat, purposeful and pretty.
The second thing I found really useful to learn how to increase and maintain a pattern was to work swatches and whilst doing the swatch really take notice of how each stitch is positioned in the row and how it relates to the row below to produce the pattern. Prior to this I used to just follow what the pattern said without looking or thinking about what was happening, just blindly following the written instrucion for rows. When I really paid attention to what each stitch did i realised i could begin to ‘read’ my knitting and then work out what stitch would be needed to extend a pattern. Often it is easier to work out what comes next towards the end of the row, extending the pattern to the left, it’s harder to work out what comes before, at the beginning of the row extending the pattern to the right. It may help to look at what would be needed at the end of the row to work out what is needed at the bginning of the row.
And then patience and practise.
I once spent 3 days counting stitches trying to work out how to maintain a pattern, i understand how you feel, I was infuriated by my inability to do something ‘simple’. The thing is, as with anything, it’s only simple once you know how, and until then it is a real challenge.
Your cardigan is so lovely, im sure you will manage these sleeves to complete it.