Ex husband ditched me, has many affairs. Can't trust anyone for second marriage

It seems like you've had a difficult and painful past with your previous relationship. It's natural for fear to take over when you have been let down once.
The positive side to this is that you feel the need for companionship again and have even taken a few steps in that direction by engaging conversation with men and also by putting up your profile on a matrimonial page.
The thing with trust is, that when it breaks once, it has the power to make you question all the other realities of life. However recognizing that one bad experience doesn't guarantee that all your experiences will be unfortunate. Think of it as a leap of faith that you want to take to be able to get on the other side where you can have a healthy relationship.
Given what you've been through, your instincts are better attuned to detecting when things are actually going wrong. Rely on them to help you avoid another unfortunate encounter. If you're having paranoid thoughts about every action of the other person, try re-directing those thoughts to a more neutral place by telling yourself, "This is just my fear taking over, let me see how I really feel about this?", and let your instinct kick in. If something is off, your gut will tell you. Intuition is very powerful that way.

If you're finding it hard to trust other people, start by taking a small step- Trust your own instincts to guide you and make the right choices.