First I would make sure you're not having a clinical depression
, which can make you feel numb when it comes to intimate relationships. That blue link will give you the sympXXXXX XXXXXst. If it's the case, then talk to your doctor about it so that you can start treatment.
If you're not depressed and you're feeling cold and uncaring with any guy you've been in a relationship with, then you need some counseling to find out what your block to intimacy is. Somehow you're projecting a past hurt onto each guy, thus treating them in the same cold way. We do this in the hope of having it be different 'this time' and working through the problem. Unfortunately, it can take too long that way. It's easier to talk to someone who can level with you and tell you what they see as your barrier.
Working out a way to be different is the next task, and that means making yourself vulnerable and trying to be more open and warm. You're going to need to make sure you pick a safe guy to do this with, and one whom you can easily communicate with. It can be a friend, even...just someone with whom you're going to let down your guard.
You don't want to go through life like an emotional cactus. It's lonely for you and painful for them. This is something a counselor can easily help you with in a short amount of time. I encourage you to try that, as well as evaluating yourself for depression.
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