What do you do when your partners have been together for 18 years and make you feel left out?

Hi! I(24f) and my partners (48f & 53M) have been together for almost 16 months now but they’ve been together almost 18 years. We’re engaged. Recently I’ve been having some internal difficulties as they both can be in their own little worlds together sometimes and I feel like I’m not even there. I know they both love me very much but it’s so difficult when sometimes they’re having whole discussions and not once do they acknowledge or try to even include me in the discussion. I know this isn’t something they do on purpose but it makes me feel left out. I have quite a bit of trauma from my past and I know they are nothing like that but I’m quite bad at expressing my feelings out loud and have a struggle putting them into words. I don’t know how to bring up this concern without feeling embarrassed about my emotions. I love them both very much but I have somewhere between mild and severe depression. I just started seeing a therapist hoping that maybe it could help me with certain aspects of my life and get my depression under control. Any advice and constructive criticism would be greatly appreciated as I don’t want this to continue to be something that is depriving my relationship.