What should I do when Im with someone and my heart is with another?

If you mean you are in love with other one but you already married. I think best way to forget about it. Because it forbidden love. We must think in our brain. Not all the time we leave our heart to decide for us.

You should talk to the person and let him know what you feel. Explain and help him to be happy in life

This is never situation to be in but its best to address this sitaution by being honest about how you feel. If youre feelings are stronger with the person you arent with then the sooner you break the news to the person youre with, the better it is for everyone. Its not fair on the person youre with and also yourself to continue being in the relationship if, your heart is with someone else. It may not be the kind of thing you want to say to someone but you cant pretend to be with someone if thats not what you want.

Question whether it's just infatuation with another, or if there's any difficulties with the relation you are in now

Talk to yourself, listen to the voice inside you. Don't ignore or suppress it. Think of both of the people and think what your heart wants. Think of why you are thinking about the other person and how long have you been feeling this way. Then make a decision. Maybe talk to a friend or write down about it. Writing can help give clarity of mind and help you untangle your feelings. We at 7 cups are always here to listen if you want to talk. :) It's important to take both of them into consideration. It might not be fair to the person you are with right now if you feel for someone else.

This is a really hard situation to be in. You could approach this by chatting to a close friend about your feelings to see what they suggest. It could also be a good idea to talk to your current partner about it, and to the person you have feelings for. However, if you feel as though you don't want to decide yet, you could maybe write up the pros and cons to either each person, or each idea of the relationship. Furthermore, it could be a good idea to think about what you may feel if the relationship with your current partner was about to end, and how you may feel if you were to start a relationship with the other person. We do not control who we fall for - love is a really complicated thing.

It is always best to just be honest. If the person you are with is not the person you want, then it may be time to move on. Sit down and have a conversation with your partner, if there are issues that you are experiencing that are leading you to seek affection from someone else, that you feel you could fix - and you want to fix - those would be worth talking about. Other than that, as hard as it is to say goodbye to a relationship, they are not all meant to last forever. Some people come into our lives to teach us lessons, and once that lesson has been learned it is time to move onto the next. You will know when it is your forever person, and if you aren't feeling that your current partner is not your forever person, you shouldn't feel obligated to stay. You have to listen to your heart, and do what makes you happy, not only for you - but for your partner too. It isn't fair to either of you to keep a relationship going if you are not fully committed to that relationship. You will both end up unhappy and you may end up resenting one another.