SC2 Campaign on Hard-- what am I missing?

I used a strategy similar to yours, and I had this same problem when I went into Brutal.

You need two things: solid macro and a willingness to work with light forces.

On the macro end, make sure you always have an SCV in production early on. Keep up on supply depots, getting supply blocked really slows you down. Once you have a good number of scvs on minerals you might want to just keep one scv building depots - but don't queue up a bunch of them, because those minerals could be used now for marines or scvs instead of being queued up into depots. The same is true of scvs and units - never have more than one unit in the queue on any building. Having none in the queue is ideal, but having only one queued up is a good compromise between keeping things building constantly (it takes a lot of attention to manage no-queue production) and not locking resources into units you won't see for 90 seconds. I tend to queue up my next unit close to when the current one is done.

Also, don't be too quick about taking gas if you can avoid it before you have the auto-refinery tech - those scvs bringing in minerals instead of gas can let you get your economy up much faster.

On the army end, it's not easy letting go of the "fortify before moving out" mentality. You'll have to do it though. Don't bother with bunkers until later (if at all), just keep marines handy. Try to break the habit of wanting 50 (or 150) marines before you poke out of your base, you can usually do the job with a lot less. Marines are often (maybe not always) the best unit you can have, especially since taking additional bases is often difficult. Just because of how cheap and quick to get they are. For example, the second tosh mission has the introduction of the goliath, but I had a much easier time on brutal when I stopped bothering with them and just used marine/medic. Collosi were hitting my marines pretty hard at the end, but not hard enough to keep me from winning with all bonus objectives in far less time than I'd needed on normal with the fortify-first strategy.

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