Certified GKH Gardening Expert
It looks like a species of Euphorbia. There are thousands, and pinning this one down will be difficult.
The fact that it is having difficulty growing, even though it is highly invasive, tells me that you have a very bad infection in your lawn. Perhaps it remains too wet.
Unfortunately, killing your entire lawn and starting fresh is ideal. Fungicides will not, likely, help you here. Solarizing the soil will be a good start.
This article will help: