Yeah, I identify with a lot of this. I withhold a lot of emotions because I can't find a way to convey the depth of meaning.
Diaries will always be difficult because you are verbalising things you normally wouldn't say - I wouldn't let that worry you too much. You have to remember to value the earnestness of the expression as well as the aptness of the delivery. Other people value this and generally those that care a lot about you will realise how meaningful and rare your expressions of feeling are. Also learn to develop more commonplace, understated language to express it when speaking to others you aren't very close to - it's better to say less and see if they take interest and ask questions, then you can add more to what you said. I don't know about you but I sometimes try to do too much and try to encompass something of great complexity and go overboard with explaining it in detail in one sitting (which people find overwhelming). Just start small and simple and build from there - it's more natural and better conveys the depth of the sentiment bit by bit.
And if you do go to the therapist, explain that you're very introspective and feel deeply but have difficulty expressing it - make a joke about it if you like. It may help him/her to know where to start and what approach to use.