What do you call a person who motivates or inspires?

My choices so far:

• motivational source
• inspirational source
• source of motivation
• source of inspiration

Being a non-native speaker, I don't know which one to use. What I want to say is that somebody has been motivating me since 2008, so the whole sentence would read:

• John Doe was motivational source since 2008.
• John Doe was the source of motivation since 2008.

Solution 1:

Your second two phrases are correct, the first two are not. It would be correct to say "a source of motivation/inspiration", but not "an inspirational/motivational source".

So in your example, you might say "John Doe has been a source of motivation to me since 2008."

Solution 2:

If you have a close personal relationship, muse is appropriate for creative or artistic inspiration, mentor for a counselor or teacher. If the relationship is not that close, you can simply say, “John Doe has inspired me since 2008.” You may also want to mention specifically what motivated you: for example, “John Doe's perseverance has inspired me since 2008.” Of the choices you offered, “source of inspiration” is best, but many writers would prefer to use inspire as a verb because it's more active and succinct.

Solution 3:

Luminary 1a person who inspires or influences others , especially one prominent in a particular sphere. (Oxford Dictionary)

Solution 4:

1. I would suggest the simple word motivator.

2. As regards your grammar (although I know you didn't ask about that), in both your suggestions:

   a. It should be John Doe has been ... since 2008 (because the 'position/activity' is on-going or only just terminating.

   b. It would sound better if you said either:

   • John Doe has been my [motivational source / motivator / inspriration / etc.] since 2008.
     or
   • John Doe has been a/the [motivational source / motivator / inspriration / etc.] for me since 2008.

   Whether you use a, my or the would depend on whether you consider that he has been the only motivator (use my or the), one of several (use a), or the main motivator or several. In the last case you could say

   • ... my main [motivator] ... or
   • ... the main [motivator] for me ...

Solution 5:

In addition to the options listed in other answers here, you could also leave out "source of" and simply call the person an "inspiration".

John Doe has been an inspiration for me since 2008.