This was brought up by another poster last week. I'll say the same I said then, you use your feet to pursue. Rarely does things stay stationary. you throw, he's gonna back step by a bit or move his head out of the way. So you'll have to use a pursuit combination.
And for reflexes, yes it is common to retaliate and react on strikes. Its the basics you have put into you when striking. Retaliate ASAP never letting more than 3 strikes get off on you.
You'll have to work on your technique in the clinch. When you do it well, you won't be tossed around effortlessly. Clinching is grappling, you're putting the opponent in a position where they're weak and you're strong.
If this is MT clinching, the game is getting them nervous and off balanced, when they're off balanced, you strike them making things much shitter for them. Its still grappling, just a different form, its not strange special foreign voodoo.
For dealing with heavier bigger dudes, you'll have to use angles. Look at yourselves like cars. He's a F150, you're a Civic or Corolla, going head to head will not end well in your favor. Take angles so when you strike you T-bone or derail them.