TBH, as parents we feel guilty about everything so I wouldn't worry too much. Easy to say I know. Some things really are for their own good though. Mine have always been a routine and they thrive on it, a friend of mine has always let her DD stay up until 10-11 at night until the parents went to bed and now she is at school there is no real routine still and she is suffering because of it. DS has always gone to bed at 7 and he is more than ready to go to sleep at this time still, which suits me. He gets a decent night sleep and we get some quiet time in the evening.
Lots of people will tell you that you are, but most of todays parents were brought up in a time when holding your childs hand while they went to sleep would have been seen as ridiculous and are we really all damaged and emotionally scarred by that?
I let my own children self-settle and sometime it took a couple of minutes and sometimes 15 or 20. They were and are fine - happy well adjusted adults who have adopted the same approach with their own (very contented) children.
I am re-married and my DH has 2 younger children who never learnt to self-settle and had their hands held while they went to sleep every night. The youngest didn't sleep through the night till she was over 5 because of this. I can't see that they have benefited from this system - probably the reverse. They are much less self-sufficient generally than my much younger grandchildren.
Stick with it - it's just a phase caused by a change in routine and will soon pass.
Obviously over Christmas it unsettled her routine. She will get back into it when she realises you aren't going to be in there all the time when she wants you to, she is probably testing you.
DS went through this phase after being a great sleeper. We would put him down, leave him then every 5 minutes, go in, gently shush and reassure him that we were still there then just leave again. He would cry but after 15, 20 minutes at the very most he would be asleep. This only went on for a few nights before he went back to settling himself again.
Personally I wouldn't give in by staying in the room, they very quickly learn that they can get you to do what they want you to do by crying. I had a friend who used to lay and hold her DDs hand to go to sleep, then she had a nightmare getting her to stop, as her DD wouldn't sleep without her holding her hand. I never started things like that in the first place because then they wouldn't know how to self settle but it sounds like your DD does so she will get it again, she just needs some reminding.
Before Christmas, my 20mo DD was a great sleeper. DH or I could put her into cot (night or nap time) and kiss her goodnight and she would send herself off to sleep. She would sleep through (and if on occasion she woke up she was very easily settled)...and when she woke up in the morning she would sit and chat to herself til one of us went in. Delightful!
Over Christmas we were travelling for a couple of weeks and she had to share rooms with us during this time - in 3 or 4 different places.
Since we have been back she refuses to put herself to sleep and screams blue murder if we leave her on her own. We have to rock her to sleep, and tip toe out but inevitably she wakes up again. If I put her to bed, I have to just leave her to cry. As soon as she wakes up (during the night or in the morning) she starts to scream - she shouts 'Mamamamamama - please please please please' - I feel so awful but if I am in the room with her she won't sleep at all. I have to leave her. So at the moment, she is crying herself to sleep almost every night (She does fall asleep after crying - it usually takes 10-15 mins). The nanny doesn't have a problem during the day at all - she goes straight to sleep.