There is such a thing as too much lapping, if that's what you're doing. If you've been at it that long, then I'd probably say, you'd better consider going to a machine shop and getting the seats done. And as was previously posted, it's usually best to have the machine shop take care of the head as a whole. Guides and seats, if not done properly, will cause you a lot of grief.
You're half way there, if you can manage the money
, and I hear panama
has very affordable tradesmen available, I'd take it to a shop.
Just my two cents.
Edit: I see you have new valves, then get the seats cut, don't just keep lapping. If you've been at it that long, you've now also changed the shape/angle of the valve itself. Bring it to a shop, or it'll just get worse.