, I feel for you. Been thro this many times, and I've had a pet put down too early and too late. For me having one put down too late because I couldn't let go was far worse as far as "guilt" was concerned for me and my pet suffered more because of it.
Here's a couple of websites that you may find helpful in making that final decision for you Lab . It's always a hard decision, and never gets easier. So sorry.....