After the initial stages of 'in love/lust' there is actually a drop in dopamine in the brain. It is possible you are seeking the 'high' of being 'in love' and when that wears off you don't know what to do. It could be depression related but you don't mention suicidal thoughts or lack of energy, and you have noted that this is a pattern. The fact that it is a pattern you've repeated suggests it may be psychological verses physiological.
There is a point in a relationship when love becomes a choice. Something you have to actively cultivate. You don't always feel the romantic love but a deeper more abiding love prevails. One too where you are accepted for who you are with full knowledge of your faults. In romantic love it is like the two of you have no faults. The rose coloured glasses people talk about. And for some people for a variety of reasons may have difficulty with this deeper love. It involves more risk, and that can be scary. Love isn't for the weak of heart.
I would really consider talking to a therapist. Even if you decide this is a person you don't want to be with, so you can break the cycle for the next relationship that comes.
Yes I have and I found that it was helpful to speak to a therapist together if the other person was willing.