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Both could be possible) you're reading too much into his behavior and 2) he gets distracted when he sees someone or something of interest on TV.
The first you can address and see if your sense of self esteem is not causing you to view his behavior as injurious to you rather than not feeling offended.
The second part, it is his approach that would make some difference. If he merely glancing at someone is one thing and another staring. The websites he frequents also would be of interest. Are these sites showing nudity/porn or are these sites where he can actually interact w/ the women on the other end via webcam?
Another thing to consider- is this an addiction of his (porn/internet) or is it once in a while if he's bored, etc?
Something else to think about would be- could your insecurity push him away even if you intent is the opposite? Your concerns are valid. Anyone can leave their partner because of a reason or no reason at all. You're not in control of what he does. Only of how you act around him and how you react to what he does and says.
If you believe that he has feelings for you, then take it one day at a time, enjoy your time together, look for fun things to do as a couple and focus on the positive aspect of the relationship. He is not perfect and that is something to accept. If his behavior is not harmful to you (if he's not cheating, lying, deceiving you, stealing, etc) then allow yourself to trust him as an individual whose in your life at the moment. This does not mean that you ought to trust anyone blindly. It simply means that you will keep an eye to see whether or not ones words match their actions without becoming consumed by anxious fear.
You would want to start noticing your internal dialog (what you tell yourself and whether or not you believe it). When you catch yourself being self depreciative, you'd want to reframe those thoughts/statements to something positive. He must be attracted to you since the two of you are a couple.
You have to change your view of yourself within not how you view yourself in his eyes. People are different. Some are younger some more mature. A person falls in love with the essence of their partner. Looks are temporary. The essence is not.
The below is a manual about how to start changing your perception. It is a form of self hypnosis, and you'd have to practice it until you get the desired results -
AUTOGENIC TRAINING - Inventor of the Chi Generator
Build Your Self Esteem Glenn Harrold