If you are having this fear, this means you are either someone with an anxious attachment, or your fears are real. If the first is true, then let your partner know how you feel, and they will most likely reassure you! This reassurance will help calm you down, and then you'll feel more secure with your partner. If the latter is true, this means you and your partner need to evalute the relationship, and arrive to a solution from there. If the solution is that your partner wants to leave you, you should let them, as them staying could lead to a toxic relationship.
This is pretty normal between you and your partner, especially after you pass the "honeymoon stage". Try your best to speak clearly with your partner, and don't argue, rather listen.
Felt. It's easy to get carried away in dreams and make-believe situations, but hoping for your mind's fantasies to become true can lead to a lot of heartbreak. Instead, focus on the present and appreciate what you do have. Learn to make the most of it and find happiness in your reality.
Depending on the stage of your relationship, this can be anywhere from low to high concern. If you just started off, don't worry too much, you'll get through as you guys are getting to know each other. However, if you guys have been far along, maybe reevalute the relationship and address your feelings to your partner. Remember, communication is key in any point of a relationship.
All relationships go through problems, and ignoring or not approaching will only make things worse. Our best advice is to just face the problem head on, no matter how daunting, because their reaction can offer a sense on the strength of the relationship. But if you are stepping away because neither of you are in a good state of mind, then that's the right choice. Wait till you both are ready to talk and have an open heartfelt conversation.