Thinking of other girl while in relationship?

I am in my 4th year with my girlfriend, and everything has been going on well.

I recently went to a wedding of a mutual friend and I had this crush on the bride when I first saw her. The bride is recently got married and is spending honey moon with her husband. FYI, I do not know the bride at all. I did not talk to her, nor do I have her contact (Facebook/Instagram). However I did take a look at her Facebook past photos etc, and other diggings.

I never had such issue before for the past 4 years, but when this happens, the feeling felt strong, and it is making me suffocate badly. I am having this tight feeling which I only experienced it before once, when my ex girlfriend broken with me, the scary feeling of being alone, overthinking.

I think about her a lot lately. and I had a lot of scenarios, pictures in my mind, of what would happen if I could get together with this lady.

May I know what can I do on this?
Thinking of other girl while in relationship?
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