Worrying/Doubting Relationship

Hi I wonder if you can help me. For the last two weeks I have suddenly starting feel really anxious and panicky. Im not really sure what triggered it but I'll have a couple of days of feeling normal and then it starts up again. I keep having these thoughts that maybe its because I don't want to be with my partner anymore. I've been with him for 4 years and everything was great, no arguments or problems. everything was just going really well as normal and then suddenly I started feeling like this and now I doubt my feelings for him. on the days when I feel 'normal' I dont have any of these thoughts and I feel happy as usual and thinking about our plans, how important he is to me etc. but when I start feeling anxious, the doubts start and they just go over and over like a broken record in my head for a day or more at a time before a phase of feeling 'normal'. I've got a drs appointment in a weeks time to try and work out whats going on. I get really panicky and scared when I contemplate breaking up with him. I know that would make me more miserable and I would regret it.