What tense should I use for describing an ongoing action which has started a long time ago? [closed]

I'd use the Present Continuous tense in the first clause, this tells your readers that you are presently occupied. For the second clause, the Simple Past seems the most appropriate, the action—starting a project—is fixed at a specified point in time.

• I'm currently working on a project which I began several years ago.

  Both forms, Present Perfect and PP Continuous, are often interchangeable and there is usually little difference in meaning. However, for actions that extend for longer periods the PP is usually preferred. Compare:

  1. I've been living in LA for a couple of years

  2. I've lived in LA for a couple of years

  3. I've lived in LA all my life

The PPC in sentence 1 suggest that the arrangement, or action , is a temporary ongoing one whereas sentence 2 could suggest that the speaker no longer lives in LA, this ambiguity is removed in sentence 3 because of the specific time reference used: “all my life”.

In the OP's example “several years” is a vague time reference, does it mean more or less than ten years? A few might say “several” means more than two but fewer than many while other native speakers might feel that several is between five and nine, or between five and fifteen or even as many as twenty. [source]. Consequently, if the OP is concerned with the most precise and correct tense to use, they should specify the number of years. The longer the period, the more I would recommend using PP with the caveat that the time reference should refer to when the action began. For example,

4. I've worked on this project since 2002

In sentence 4, the speaker is still working on the project. This action may or may not be continuous, it may or may not have been interrupted, only further context will clarify.

Unfortunately, I don't think there is more specific way than "have been working' to indicate the project is ongoing.

The only thing that comes to mind for me is "have continued to"

I have continued to work on this project for several years.