I can't stay faithful to one man

I can’t remember when I’ve had only one boyfriend— I’ve always two-timed, at times having three relationships at one go.
The men do it all the time and if my lovers don’t know about each other, where is the harm? 

I’ve now met a very responsible man and we are talking marriage. I need to change my ways, but I have no clue as to how to stay faithful to just one man.
Any pointers? 

For now, the fact that you’re in love with Mr. Right should keep you faithful, especially with the prospect of marriage.
You’ll be so happy with him you won’t want to rock the boat by misbehaving.
When the excitement dwindles, however, you may find yourself wondering if he’s enough for you and might be tempted to fill in the gaps.
When that happens, instead of finding comfort in the arms of another man, you must hang on to face your problems. Being in love is said to be a state of madness when lust and adoration is overwhelming.
But that can’t last and what’s real and worth holding on to is friendship, caring and shared hopes for a future together as well as sexual compatibility.
(Bunmi Sofola)