Big Hugs Jap1990, unfortunately it's just something you have to try your best to let go of and move on from. He is no longer with that person, he chose to be with you which is what means the most
My husband is 9yrs older than I am, we met on my 18th birthday, so he was VERY experienced - had been in a previous 8yr long live in relationship and it took me a long long time to deal with things but we were open and honest with one another about everything and some how it just got easier to the point I don't think about it much any longer.
We have been together 17yrs this year

Don't get me wrong, sometimes I still overthink things and allow my mind to get the better of me but I have to remind myself that it is HIS past, not mine and he chose to spend the rest of his life with ME, not her
I don't know if this helps, but that's my story

All the best! Feel free to ask me any questions!
K.