Overheating problem?

Car was hard to start for approximately 4 months before it started to run hot. I was able to turn the heater on high and go about 15 miles before it really started to go way above the halfway mark. Still was able to let cool add water and ho another 15 miles to home. Turned it off before it reached the red area. Started with a thermostat, same issues. Then bypassed heater core, same issues, new radiator same issue, new water pump same issues, new radiator hoses, new idler pulley and new belt, same issues, another new thermostat, same issues. There are no signs of coolant in the oil, but still haven't gotten temperature hand over in red area. But has started to run rough when the first crank. Then levels out. No sign of leaking water. When stop now it sounds like water maybe boiling but not losing water that can be seen. Now it reads hot at 2 miles from home but let it sit for 15 minutes and good to go 2 miles. I actually opened the radiator about 6 minutes after stopping the car and it did not boil over. Now I've quit trying because I haven't the money to keep guessing. I've never worked on a car before, but my husband passed, and I'm forced to try and fix it. Next, I was going to buy a coolant temperature sensor as soon as I can buy one but for it. But not sure if I will be wasting money, I think it's like a $100.00 sensor. Any ideas what's wrong or should I go ahead and plan on head gasket leaking? Oh yeah. And it's blowing out more white smoke when running than usual. Please help me because I am mechanic illiterate.