You need to track your time and limit all that you put in. I use a spreadsheet that I put the times I start, take lunch, and then it calculates when I need to stop. So if I take a 37 minute lunch, I can set a timer on my phone to go off and remind me. I've also setup a automatic timer to switch on a lamp that starts at roughly an hour before I need to log on and cuts out at lunch. After lunch I turn it back on util I'm off. I only go into work mode with that light on. Simple but effective. Then you have to make other things worth doing, like I'd rather throw a ball for my dog than keep working. Or a hobby that gets you away from the PC, knitting would work and easy to get started.