Hi trueunicorn, and welcome.
I'm truly sorry to hear that you are going through heartbreak and confusion. I hope I/we can help.
Firstly, I have often found that people who are extremely quick to declare highly intense feelings (to me, quick is before even a month has passed) are often likely to develop and declare those feelings for others just as quickly. Sometimes we can meet a person and just instantly be bowled over by them and enter a lifelong partnership, but in my personal opinion, huge declarations so early in the game can be a sign of a lack of control over one's emotions.
While I cannot tell you why he drifted off so suddenly, I do recognise the signs of a person who seems to fit into the above category. Additionally, some people love the chase, but once the other person feels safe enough to reciprocate with the same level of intensity, the originally intense person becomes turned off.
The only way to know for sure would be to ask him. I would personally advise this as good practice, because if he does give you an honest reason, you are bound to learn something that could teach you something about yourself (even if that's not to jump in too soon).
The other thing I would like to pick up from your thread is that it sounds like this was the first guy you've dated since opening your relationsip up? It is extremely common to fall head over heels into NRE
during the early days of poly, become completely overwhelmed by these new, powerful feelings, and become utterly devastated when it ends. NRE can be particularly prevalent when one has been in an existing for a long time and then meets a new person. This can also bring feelings of confusions, guilt, etc. The more you understand these feelings and learn to balance new relationships, the more perspective you can retain.
There does seem to be some work needing to be done in the communication realm. I feel that it is completely acceptable (actually, desirable) to provide support for - and ask for support from - our existing partner(s). In fact, I think it's vitally important that someone you live with and share your life with should know when you are heartbroken.
What advice do you have? And how do you protect yourself from falling for the new person? Is this a rookie mistake or is this a known side-effect of poly relationships?
Having practised both 'polysexuality' and polyamory, I can say that I was
able to avoid falling in love with other partners, but this took extreme self-restriction and constant awareness. If you choose to go down that route, I would recommend choosing play partners you have a low connection with, and/or ensuring that the texting/communicating is kept to an absolute minimum, such as only getting in touch to meet up. If you start to become too close to a new person, you will have to step back or end it. You should also be firmly clear that you only want sex and fun, and if they begin to express attachment, you should not encourage it. However, if you don't want to restrict yourself (or simply don't restrict yourself) love can of course be inevitable! In this case, you may choose to become polyamorous and open to the possibility of love. Whether you choose to do that is absolutely down to you - you must do what feels right for you!
My honest advice would be to reconsider your situation, agreements, paradigm and communication. There is every chance that your partner may fall for someone else at some point, and that you could again. It might be the time to be candid about that. If you are absolutely certain that you're not up for polyamory (i.e. loving others), and you really feel that you can't tell your partner, the only thing you can do is try to heal and avoid the intensity with future play partners.
Whichever path you decide to take, I wish you luck!!