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What you are talking about is known as the continuous/progressive aspect in English.
In English, the present tense and the continuous aspect can be used interchangeably. Both are formed using a conjugated form of 'to be' and a present participle (which looks like a "verb" ending in '-ing'). Note that in English we call the second component a present participle. It is not a gerund. In English, a gerund also ends in '-ing', but it acts as a noun, as in 'waiting is not fun'.
The Spanish equivalent of an English gerund, the word that acts as a noun, is the infinitive, not the Spanish gerundio.
The English equivalent of the Spanish gerundio is the present participle.
Now, with that understanding, the continuous/progressive aspect is used differently in Spanish than it is in English. As you wrote, it means doing something at that very second in Spanish. It can also be used to describe a habitual action (as in, 'I am always waiting'). In the past tense (using 'estaba' or 'estuve' and the gerundio (there are two possible past tenses)), you are simply describing what you were doing in the past (when something else occurred) or you are giving details to someone else about what was happening (a progression of events) or you're describing a habitual action done in the past. You've switched from the present continuous/progressive to the past continuous/progressive aspect.
I hope this explanation helped a bit.
By the way, the Spanish present tense verb 'espero' can be translated into English as 'I wait', 'I do wait', "I'm waiting" or "I'll be waiting" (describing a near future event). To the Spanish mind, 'espero' doesn't mean the same as 'estoy esperando'. The former is a conjugation in the presente indicativo, while the latter is an example of the 'presente continuo', una forma del 'perífrasis verbal' estar + gerundio. The 'presente indicativo' and the 'presente continuo' are not interchangeable.
In the past, the verbal periphrasis is called the pasado continuo. Whether you use 'estaba' or 'estuve' with the gerundio is a matter of whether a certain time frame is perceived.
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