So, just for 'I', you have:
era
estaba
fui
estuve
When you talk about how you were feeling in the past you would use 'estar', since feelings are temporary. So for 'I', you would say 'estuve feliz' or 'estaba feliz' to say 'I was happy'. Talking about a party last week you could say 'estuvo divertido', to say 'it was fun'.If you talk about a past profession you would use 'ser', so you could say, 'fui un doctor, pero ahora soy abogado' = 'I was a doctor but now I'm a lawyer'.
To say 'I was going to' in Spanish, you use 'estar'. So you could say something like 'estaba a punto de ir' = I was about to go. To say 'I was accustomed to something' you would say 'estaba/estuve acostumbrado a algo'.
Now, 'estaba' and 'era' are both examples of the imperfect tense in Spanish, which as I've said is used to talk about actions 'in progress; in the past. However, you also have the option to use the imperfect of other verbs to say the same thing without even needing to use the verb 'be'!
For example:
Estaba cocinando o cocinaba = I was cooking
Estaba pensando o pensaba = I was thinking
Estaba yendo o iba = I was going
(Notice that in none of these example I use the pronoun 'yo', because since every verb conjugation is different there's no need)
So you can say 'estaba a punto de ir' to say 'I was going to go', or you could say 'Iba a ir'. If you don't like either of those constructions you can also say 'quería ir' which means 'I wanted to go' but is also commonly used. :-)