I read your post and had to reply. I don't have a happy ending to my story as you can see by my signature but wanted to share my experience as I am sure you can relate to some of it.
I had a BFP in late March. My numbers started in the low 60's. I kept having to go in every 2 days for b/w. It was an emotional roller coaster. You're right- much worse than the 2ww! My numbers weren't doubling like they should, but they were rising. My RE kept saying to stay cautiously optimistic, but it was hard. I was feeling exactly how you described, trying to stay positive but wanted to be realistic. Kept trying to be mentally prepared for the worst. Then we went in for an u/s and we saw the gestational sac. It was the smallest black whole on the u/s screen I had seen. My hopes were up again. I thought this could be it for us. Unfortunately, my Hcg levels were not where they should be at this point. My RE recommended we stop the meds as to not put my body through further stress. He told us this was a miscarriage.
I was okay when he told me. (I was at work at the time). I thought, "Oh good, I'm mentally prepared. I can handle this." But boy, how I cried when I got home. My husband held me for what felt like hours as we grieved together. Now, looking back, I wish I could give you some "words of wisdom" of how to better prepare yourself mentally. I thought I had figured that part out. But I don't think there is an answer. If I had to go through this all over again, I would definitely stay cautiously optimistic. However, upon hearing bad news, I think I would cry just as much and feel just as sad. There's no mental preparation that I think will make it any easier. I do want to say to focus on the positive. If this doesn't work out, then at least you know you CAN get pregnant. When hearing the first BFP news from our RE, I remember how shocked and excited I was because up to that point, I didn't know if our fertility problems laid with the embryos not being able to implant. Recently, we met with our RE, and he also stressed the positive news of being able to get pregnant. We have 3 frozen embies left and will try again in July/August. I'm hoping third time's the charm for us.
I wish you the best!! And just because I don't have a happy ending doesn't mean you won't. Please let us know how things work out. Keep up the PMA- as much as possible, right?

Good PMA is so important through our IVF journey.