How do you know when the relationship is no longer worth fighting for?

7 yrs and many times I found evidence of cheating.. He does not support our kids financially and I really don't want to touch him physically anymore just based on the cheating... it's a turn ff to me, and all I can think of is "did he do this the same way with me, as he did her?" We lived together 5 yrs, untill I just got tired of supporting all of us, just for his little chump change to end up getting garnished to go to his other 2 kids child support. He now lives with his mother and I moved me and our 2 kids a block away from them for the help with the kids, and in hope that we would have a better relationship... It started to get better, and we spent everyday together, but I kept noticing him hiding his cell phone away from me, (i found him cheating before by going thru it)...He sleeps with it in his sock, and the one time it dropped out, It kept vibrating and the msg was from a lady sweettalking him and commenting on their evening the night before. That night , he wouldn't answer any of my calls. I knew he was still hiding stuff from me, but if he was really trying to give our relationship another shot, why would he still be screwing up intentionally?? I want our fam to be together, but the cheating thing is just killing me. I can't even try to be a better woman to him, when all i see is a cheater/liar? At what point should i throw the towel in? We've talked about this before, and all he does is deny, even when I have evidence in his face. I really want my family together, especially with 2 boys, they need a father in their lives.