Let's take a scenario in which I had practiced brahmacharya for about 1 year or so... Now if I involuntarily got attracted mentally by a girl..and I praised her beauty. But immediately felt wrong and held back myself from that thoughts Does that break my brahmacharya?? Will my one year of hard work go in vain??
See, this is from my own experience of Sadhana I am talking. Do not try to control over your thought process about this worldly affairs. Until you realize supreme, when senses come in contact with objects they like, when mind's attractive things you encounter, these thoughts bound to rise. So do not worry about these thoughts arising in mind. As long as anything is restricted to just thoughts, it's fine because we're in Kaliyuga, and the culture, the way we dress up, everything has changed. So you are not to be blamed.
Your focus should be on God-realization. And be focused on things which will make you realize God, and just keep moving in that direction. That will gradually release you from all bondages. When you do that, automatically negative forces which block God realization leaves. Move on what is essential, automatically non-essential leaves you. That's why in Tamil they say"ஸ்ரீ தேவி வீடு வாசலில் வந்தா, முதேவி பின்வாசல் வழியாக போய்டும்." - Focus on what you need to do in life, rest of the things will automatically fall in place
According to the Daksha Smriti (DS) sexual intercourse can be of eight kinds.
See the following verses:
[A Yogin} should always preserve his Brahmacharyya [celibacy]; Sexual intercourse is of eight sorts : -viz: thinking of a woman, talking [about it] -dalliance with a woman, looking [at a woman with an impure desire] speaking to her secretly, determination [for holding a sexual congress], persistent endeavour [for doing it] and the actual deed. The learned hold that these are the eight divisions of sexual intercourse.
Any of these eight acts are considered as against celibacy.
So, even thinking about a woman is in violation of celibacy. But IMO, it is here talking about thinking sexually about a woman (i.e having amorous thoughts about a lady) and not about just thinking.
In your case, you simply praised her beauty in mind but did not have sexual thoughts about her.
So, IMO, your celibacy vow is still intact.