Writing Cantonese characters on the computer can be quite confusing and it seems most times the characters we foreigners learn are different to those used by native speakers =P
Personally, I use the character 係 (haih) when linking two nouns with the English meaning "is". I use the character 喺 (hái) when stating a location.
Examples:
我係學生 - I'm a student
我喺屋企 - I'm at home
As for moods, you could try using other verbs. For example you can say 覺得 (gok dāk) which means "feel". Or you can just leave out a verb and just stick with 好 (hóu).
Examples:
我覺得好開心 - I'm so happy
我好開心 - I'm so happy
In the beginning when learning a language, as you've mentioned, most of the sentences you make start with "I am" or 我係. So, to get past this "boring" stage you could start trying to make your sentences more complicated by using more adverbs (such as 今日 "today") or impersonal phrases like 有個人 "there was a person". This sets up a bit of background and and introduces the topic and then you can mention what you did.
Example:
今日,有個人同我講廣東話,我覺得好開心 - Today, someone spoke to me in Cantonese and I was so happy
I hope this helps you avoid saying 我係 all the time =)
加油!