Dear Masmal, thank you for your question.
Feelings are a very important part of life. We all have emotions and the Bible speaks a lot about things such as love and joy. Jesus showed emotions - weeping when he heard his friend Lazarus had died (John 11), being agry at the way the temple was being used (Mark 11), showing compassion (Mark 8). So the Bible views emotions positively and not as something to be ignored. In fact a book in the Bible (Song of Songs) is almost entirely given to the feeling of being ‘in love’.
However, emotions should not be the primary thing that drives us, now should they be the means by which we determine the rightness of a decision because they too are capable of leading us to sin. It is the Bible that we must trust and obey, not our feelings. If we doubt God’s love for us, it doesn’t mean that he doesn’t love us. So too in relationships. If people are married and you start having thoughts of doubt then the Bible commands us to remain married. If you aren’t married, then there is more freedom, however it doesn’t mean that at the first hint of doubt you should leave a relationship that you are in.
It is normal to have doubts from time-to-time. The thing to do is analyse why you have doubts. There may be legitimate reasons or they may turn out to have no basis in reality. It would be good to talk with a Christian you trust as to whether the doubts you might be having are legitimate. As to what Christians should look for in a relationship, the Bible says more about the character of people than the ins-and-outs of a relationship. So Christians should be looking for people who are growing as Christians, people they can encourage and who will encourage them. That will mean they are submitting to God and saying ‘no’ to sin.
As to being ‘in love’ or just ‘love’, I don’t think there is much of a difference, except to say that what the world often means by ‘in love’ is infatuation and lust whereas the Bible speaks of love as selfless active care and concern for another’s wellbeing - ‘this is love, not that we have loved God but that he loved us and sent his Son to be the atoning sacrifice for our sins’. 1 John 4:10. In other words, love is more about sacrifice and service than a warm feeling.