Your number one problem is LOW SELF-ESTEEM.
WHY do you ALWAYS NEED
a man? You were unhappy with your son's father, so you left him for this guy. Now you're unhappy with this guy, and you want to leave him for your 'ex.' BIG MISTAKE! Whatever it WAS about the 'ex' that made you leave him is STILL GOING TO BE THERE. Only NOW you will have even less tolerance for whatever it was than you had the last time you were with him.
STAND UP ON YOUR OWN TWO FEET AND BE A WOMAN. BE AN ADULT. Quit looking for a man to be your crutch, you don't need one. You should NEVER have a man in your life because you "NEED" him, you should have him in your life because you "WANT" him. Because he complements your life (and your son's.) Because you enjoy having him there, BUT you could certainly be just fine WITHOUT him.
* Don't have an affair. It's stupid, it's selfish, it's short-term 'make me feel good' thinking instead of long-term 'what is in the best interest of my son and myself' thinking.
* Get into Individual Counseling. Find out how to fix YOU. If you don't fix you, you will continue to have these unsatisfying relationships one-after-another...all the while dragging your son through the infinite number of messes you create.
* Don't be afraid to separate from your husband if that is what your counseling leads you to. YOU are the breadwinner for you and your son; and you have done it successfully for the last 4 years. As 'turnera' pointed out:
YOU pay the bills. YOU handle the housework. YOU raise your son.
You've ALREADY BEEN DOING IT for the last 4 years, but you haven't given yourself the CREDIT for doing it. You don't NEED a man to make it. You're doing just fine so far for you and your son.
You will have MANY opportunities to find APPROPRIATE male role models for your son. Don't settle for just anybody because just anybody may model behavior for your son that YOU DON'T WANT HIM TO HAVE as a grown man.
Get into IC. Get into IC. Get into IC. Get into IC. Only with a CLEAR MIND and thoughful goals will you EVER make better choices and a better life for you and your son.