Hi Zrinka and welcome to the forum,
I think selling something you know well and have a passion for can be both a good and bad thing. Product knowledge is gold in any form of sales. The more know about the products you sell, the better you are able to market them and provide valuable customer support for them. Where it can be a bad thing is if you ignore all good business sense when the indicators are bad, and then just push forward with a products because of your own belief.
Has to remain a balance. Your knowledge is a plus, just make sure you use a business approach to anything you sell and be guided by that.