I think that if apologizing will help you move forward, you should go ahead and do it. I always think it's a shame if a genuinely-felt apology is held back.

I don't think anyone's telling you to stuff your feelings about it inside (although I don't want to speak for anyone else). I think the concern is that you'll make this the focus of your therapy. But that doesn't sound like what you're saying right now. It seems like apologizing will help you step AWAY from the issue, rather than getting stuck in it. If it were me, I'd probably try to approach it that way -- that is, saying sorry as part of deliberately setting the issue aside, so the focus can remain on what sort of stuff this brings up for me and how it can help me learn about myself.