Hey @CassM
I hope you are feeling better today. I had a relationship that lasted 5 years and with you 11... I can imagine how attached you two must have been. Though I do not know what is a good advice to give you but I can tell you what helped me to get past the pain. I know its tough because as I describe it to others... I feel like I grew up with this person.
One of the happiest feeling that I experienced when I broke up with my ex was that how free I was now. I no longer need to think myself as "we/us" and I can focus on what I want in life. Take 15 to 30min walks and discover what you want in life now that you are no longer in a relationship. You can now go abroad and work/study, you can pickup that hobby that you've always wanted to. You can go visit your friends any time and stay as long as you like. Discover who you are and pursue your dreams.
One of the other important things is that I made peace with that I care for her and if she is happier with this other man (they are married now) then I should be happy for her. Nothing in life is ever forever. People change and feelings change, I shouldnt and cannot expect otherwise. I don't own her and I am sure she did not change on purpose... its just the way life is.
So be happy for your ex and don't feel down. You have friends, hobbies, career, family, studies, sports, travel, volunteering to discover and grow stronger bond with. This is an exciting and happy time for you... enjoy it.
Take care.