Dear Indeed,
Welcome to the forums.
Intrusive thoughts can put a strain on our relationship because it always puts you on edge. You're suspicious that your partner is cheating or lying and most of the time and that prevents you from fully being invested in the relationship.
I understand that you want your partner to comfort you regarding your suspicious, but it's a bit of a stretch. Think about it this way. If your partner had approached you accusing you of cheating, lying or manipulating you, would you be more inclined to shrug it off and comfort her, or would you get more depressed and break down thinking that your partner doesn't trust you? The latter to me seems more likely.
As I see it, the best way for you to deal with your own intrusive thoughts is to examine why you have these thoughts in the first place. You have not mentioned that it is based on something she did before (e.g. did she cheat before?). If there is no basis for this thoughts, it's likely your thoughts are just a manifestation of your fears. Do you fear that good things will not last? Or do you fear being betrayed? It's worth taking some time to reflect on this.
Your situation is made worse by the fact that your partner is also dependent on you for emotional support, and that will wear you down as well. Go easy on yourself because you're in a tough situation. I can't comment on the Snapchat messages, but I can say that we sometimes see more than we should when we're not in the right frame of mind (e.g. if we're convinced someone is tired of us, the lack of an emoji in a message might be enough to convince us that they are in fact tired of us. In reality, it could have meant nothing).
Take care,
M
