I will try to keep it short.

I've noticed that the girls too often see me as a guy who is a "probably having interest in lot's of girls at the same time". Well, of course that is the case in many situations, but not all the time and I find it interesting that they tend to classify me as such person even if they are the only ones that I'm hitting on at that moment.

As a result they consider themselves as one of many that I am interested in and therefore it is really hard for me to progress any further.

My questions:
1. What possibly could make them think of me like that?
2. How to reduce the chance of them thinking of me like that?
3. What are your ideas for dealing with the girls who already think of me like that?

Are you interested in short term or long term?
If it's the second, emphasize you're looking for something special.

It it's the first: you don't have to apologize for dating many girls. You don't have to be sorry to have a sex drive, in fact, many people would like the flirtatious lifestyle if only they had the guts. Own your lifestyle. I'd just say I like to get to know many people and ignore the 'tone' of judgement.

You may reduce the chance they think like that in the first place by also hanging out with guys and female normal friends and being discrete about girls you have hit on, if you currently are not.