Cancer man: Why is he adding cute girls when he said he's in love with me? Should i worry?

[Been There]

If you want to use astrology to understand another person, use real astrology.
You are using "Sun sign" astrology (his Sun is in Cancer).
Which is only 6% of his entire astrological profile, a small part .. and it's not even the part of his birth chart that indicates his social or romantic behavior. Sun is ONLY how we need to relate to ourselves IF we are going to feel whole and complete.
His Sun sign will not answer any question you have about his relationship style.
Anyone who talks about "the Cancer man" doesn't understand astrology or how it works. They have been scammed by commercial Sun-sign astrology.

So if you want to actually use astrology go to astro.com or alabe.com to do up his free birthchart, entering the day-month-year, hour-minute, and longitude-latitude (or city-state-country) of his birthplace. This will give you HIS chart.
700 million people have Sun in Cancer, but they are not all the same. Only the personal birthchart will tell you about an individual.

From what you say:
He's not a faithful man.
If you break up with him now, you will no longer worry. If you stay with him you will feel the nagging pain of worry for the rest of your life.
You have to care about your own happiness enough to choose wisely who you love, and rejecting those who will not be good for you.

Being "in love" is NOT love.
It is infatuation, and never lasts longer than 2 years or so.
Infatuation is a fantasy that involves the release of brain chemicals that make us "high" over the other. It is being in love with being in love, although we mistake it as loving the other person. It isn't.

Real love takes at least 2 years, of being IN an understood relationship, before it even starts to grow. And it feels nothing like infatuation.

Expect infatuation to die out.
Be surprised if there is anything left to work with once the infatuation has died.
And hope that you are the first to fall out of infatuation, because it hurts like H if they are the first ones to go back to reality.