Although it's hard to change the way that you've been putting your child to sleep now that it's such an ingrained habit, it's not too late - you can do it. From when they were tiny, I would let my babies go to sleep in my arms and then I'd put them in bed and that's how they were used to going to sleep too. The way that I transitioned them was:
- Make sure they have a nice little routine (bath, story etc).
- Turn the lights off, try to keep noise to a minimum and tell them that it's "sleep time" now.
- Once they were finished their milk, I'd put them down in bed and sing them a little song. I wouldn't pick them up again because it was time for them to lie down and sleep.
- Once the song was finished, I'd give them their special cuddling toy, tell them night night and leave.
I found that my kids would not sleep if I was there in the room with them (I tried that, it was too exciting). Now, the trouble is that your toddler is used to being held and gets agitated if you leave. I find that my kids get really mad and they flip out if they don't feel that they want to go to sleep so what I'd do is I'd leave them for a minute or so and then go back in and see how they are doing, say goodnight and re-iterate that it's "sleep time". Then I'd leave them for 2 mins, 5 mins and increase the interval and eventually they'd get the idea and go to sleep. It's pretty much the hardest thing to have to do to hear your baby crying, but it's important for them to get a good sleep every night so that might make you feel better about it. It can be very upsetting and difficult for the parent too so make sure you have your partner there to support you when it gets tough.
Another technique you might want to try in your situation, if leaving her is too much to start with, is have a story with her in your arms, and make sure she gets a good amount of cuddling before bed, then put her into bed. Instead of holding her, perhaps just try holding her hand, and then transition to a little bit further away gradually - first by being in a chair or something nearby, then move the chair further away and eventually out the door. If you hold her hand, you can stay with her but she will learn to go to sleep on her own. She's already two and a half so she understands quite a lot, so tell her that you're going to put her into bed and that you'll stay with her.
Just a side note, I see there that you are thinking of putting her in a "big girl bed" (cot with a low side). I think you'd be crazy to try that at this stage. My experience with that is the minute you remove the side, they start running out of bed to come and see you instead of staying put. My daughter (almost 2 1/2 yrs) cannot be trusted to do that at all yet but she is usually pretty good about lying down to go to sleep under normal circumstances in her cot bed. My older daughter (who is two years older) did the same thing and it took until she was about three before she'd stay there at night. She still has her moments to this day about staying in bed but I usually give her a toy to play with and she just falls asleep playing, and I have no problem with that.
Best of luck!