Hiya Jim, welcome to the DhO.
I get to access then either get eye/headache (over-focusing?) or just stay at access for a bit then loose concentration.
I think you're right about over-focusing, when you hit access concentration stay with the breath but hold it lightly rather than zeroing in on it tightly. It takes a bit of trial and error to find the balance but you'll find that is easier if you're not fighting with it and
trying to stay with your object. Let attention rest on it naturally and just keep bringing it back
gently if you catch yourself wandering off, your breath counting is a great way to do this. Remember that every time you realize that you've followed a train of thought, or got distracted, you're immediately back to being aware of your object again which is where you want to be.
Once you've gotten access concentration, begin to pay attention to the pleasant feelings arising in the body but, again, don't
try to do anything with them other than observe them as they are. This will lead to an increase in those sorts of feelings and if you can continue to focus on the jhana factors arising then you'll find 1st jhana showing up quite naturally.
The way I would go about dropping the tightening focus and over-exertion would be to just accept that what's happening is what's happening, that jhana will naturally arise if the conditions are right for it to do so, and that thoughts of "I must get into jhana" or "I really want to experience jhana" are not actually required or even helpful. Thought will arise but you're not trying to stop that from happening, accept that they're there and acknowledge them without fighting with them or trying to change them, stay with your object and it'll happen.
Hope that's of use to you but feel free to ask whatever, there's plenty of people on here with really good concentration skills who can help out so get as much info as you can. There's a knack to it but, once you get used to it and are familiar with it, it's possible to enter jhana at will with a bit of practice.
Tommy