I am still very new to poly so if my terms are wrong please forgive me.

I have a partner who I have been seeing for about 6 weeks now and my husband has someone he sees off and on but it’s not regular and they don’t really chat much. My husband met someone new this past Monday at a munch event off fet and they went out on Wednesday for a date and have been none stop texting and making plans and even both went to get tested so they can Be sexual and safe. I am having a lot of feelings and issues that this is moving so fast because I am demisexual and it always takes me so much time with dating and wanting to be with people so I’m not sure how to deal with any of this. I would love it is there was some sites or books or even videos that could help me work on my feelings because I don’t like how I am feeling with them moving so fast. I am feeling like I will lose my husband and that’s hard on me. So any help would be great.