All of us have been hurt by someone at least once in our lifetime. We trusted someone or that something would work out and then we ended up dissapointed. Some of us have been dissapointed more than others.
It is normal to keep your guard up after being hurt but if you keep it up too much you could be missing out on a lot. You will start to have fears and you could even ruin something good with someone amazing.
The law of attraction states that what we put out there we recieve. So if we believe that no one will love us or every guy will cheat on us then we are adding more of that into our lives. Some of your friends may even tell you not to get your high hopes up because then you will be dissapointed. Don't listen to them. Love with all your heart because love is one of the greatest feelings in all of the world.
Always remember just because one person didn't want you does not mean that someone else won't. It is possible to meet someone who will want you and only you. Love, get excited and be ready. It may be hard but falling in love is worth the risk.