How can therapy help me with untreated anxiety and depression?

Therapy is a great place to have support and learn to build up your confidence.  It is nice to be validated in your feelings and have a place to go and vent so you aren't carrying those thoughts and emotions with nowhere to put them.  

You have mentioned not having an official diagnosis but have been having symptoms.  Getting a confirmation of something is also helpful because you do not feel like you do not understand what is going on and you can start working on the right things in therapy to manage those symptoms, learn some coping skills and ways to manage emotions. Therapy also helps people understand that they can do something to improve their situation. That leads to changes that enhance healthy behavior, whether it's improving relationships, expressing emotions better, doing better at work or school, or thinking more positively. 

You have a wedding in the next year or two which is a very exciting event and worth getting a lot of focus and attention. You do not want it to be overpowered by the stress of trying to manage all that you are without any other supports.  Therapy can be very helpful and I typically recommend people at least give it a shot to see for themselves if they like it. If you do not like it, you can always stop or try to meet with a different therapist as there are different personality types.  There are therapists that specialize in specific issues and that really helps for the care as well as your treatment path. 

I wish you the best of luck in finding someone and hope you are able to manage the stress and it helps give you mental strength to continue being that caregiver as well.  You do not need to be carrying those thoughts, emotions, feelings, and stressors without any support.  There are people that would love to help you.