If you give him the impression that you are nothing without him, you may end up alone, but if you behave the other way, this won't happen.
Show your lover that you are an independent woman, that you can take care of yourself from all points of view (financially, psychologically, sentimentally, even sexually) and life does not evolve around a man.
2. Don't let yourself be intimidated
You are in a club and another woman flirts with him? Make sure he doesn't respond and let her know he is not available. Whenever another woman appears, make sure she knows he is taken. Everything needs to be settled nicely and with dignity. Don't get paranoiac and do not make scenes.
3. You with yourself
Make time for yourself. Don't always be with him otherwise you will get an addiction. Go to a park, read a book on the bench, go shopping, see what is new, what is the latest fashion, go out with your friends, visit your parents, go to a movie. Invite him to do the same - he will appreciate this.
4. Free time for him
There is no week without a football match. If you can't afford buying tickets for him and his other 5 friends, you can ''rent'' them your apartment for 90 minutes (well, maybe 3 hours in all). Stuff the fridge with beer, prepare 3 bags of popcorn and go for a walk with your friends. He will be grateful for having such a girlfriend (and the others will envy him).
You don't have to do much to keep your lover. You only need to pay attention to the details that will make him appreciate you more day by day.