Change From Citalopram 25mg To Sertraline 50mg

I'm 35 years old and have been on citalopram (10 to 25mg) for about 13 years. It has helped me in general but never stopped my panic attacks completely. When I had a bad time I had to increase my tablets which always made me feel worse before better. What I find is that I feel emotionally numb. I don't get excited about things like for ex going on holiday. Instead I see it as a hurdle like I have to spend hrs on a plane which I really dislike, what if people are going to be loud, moving my seat... And what if I feel totally awful whilst I'm there... That kind of thing. I start shaking, freaking out, vomiting, can't eat, function, terrified.... I saw a doctor a while back who said that she didn't think this medication was working for me anymore and that it could be normal as I've been on them for so long. It is now affecting my relationship as my other half always says I'm not happy or if I am I'm not showing it. Anyway, I decided to see my GP yesterday who told me to try sertraline. However, i have been on 25mg of citalopram for the last 5 months and have not had any severe panic/anxiety attacks but I can tell that it's still there waiting to erupt. Now, I am petrified to change my med!! What if I'm gonna feel worse or awful like I did last year! anyone here changed cit for sertraline after a long time? Am I gonna get withdrawal symptoms from cit before I get used to sertraline?? If you have been through it I would be very grateful if you could tell me how it went... I just want to be happy and live life to the fullest. I feel I haven't been able to in the last 15 years. I'm supposed to take my last citalopram tonight and go straight to sertraline tomorrow.