When to let go....

ok, this is some Agony Aunt ish. When do you let go in a relationship and just decide enough is enough. Throughout the years, I've watched my friends and I go through relationships ad breakups and as we get older, it seems to me that we have the tendency to tolerate alot of things and hold on for so long. Lately, my friend has been going through some stuff with her BF and sometimes I just wonder, why the F*** do we hold on for so long?
maybe its cos we getting older and we feel that marriage is around the corner and when you're dating someone, you picture yourself being married to them, so we just stick around hoping for the better.. hmmm I wonder
Am just to optimistic about guys now, its like I've lost the feeling for them( "wahumaneh am lesbo deh..ahaaa",(didnt say am lesbian) or that I dont care anymore, but I feel that at this day and age, when am beautiful n intellingent, I can get any damm guy I well please, just like there are plenty girls around there are also plenty guys around, and there are some good guys so ladies please dont stick around in situation that you honestly know you worth more than and deserve more.