Marriage

Bismihi ta'āla

Yes, it is permissible to marry a Sunni, as long as the person of the other madhhab does not affect your belief in your madhhab.

And Allah knows best

Bismihi ta'āla

I am not sure what madhhab you and your husband are, as there are different laws in the Shi'i madhhab regarding divorce. 

Please refer to your local scholar for information.

And Allah knows best

Bismihi ta'āla

If one feels they are "unprepared" for marriage, then should work on preparing themselves, and make that their fundamental priority. Nothing is better than getting married.

Pursuing temporary marriage is not going result in long-term solution, so it is good they do not wish to pursue it. 

Sexual lust is natural, given to us by Allah ta'ala for many benefits. We just need to regulate it, and control it when needed. We are all prone to becoming weak in our self-control, and Shaytan could overcome us. However, with jihad al-nafs, and resisting haram urges, one will see wonders. 

Strong focus on acts of worship is one effective method of coping with lust, especially prayer and connecting with the Lord. 

Avoid anything that leads to rising of lust, like what you see, or hear. Try to fast, as the Prophet (s.a.w.) advised.

Occupy yourself with various activities. Try not to be alone, and interact with family members more. 

The element of dua is also very important. Immerse yourself in supplications, beseeching Allah and imploring Him to keep you pure, and grant you a righteous spouse. 

And Allah knows best

Bismihi ta'āla

Involve parents, elders, or your local scholar qualified for this to intervene. Have a meeting, try to find ways of reconciling. Try to exhort all effort in amending the relationship, so that if your only choice is divorce, you will do so consciously and responsibly. 

Please watch this, as it will hopefully benefit you in making your decision the right way.

https://youtu.be/vRlB_LkIuVI?si=T7oAoMRv3aFvjHXJ

And Allah knows best

Bismihi ta'āla

I do not know your circumstances, and you might both have a valid reason as to why you have not got married yet, but I do not really understand what "respectful long-distance relationship" is. Does it mean you are married, or in a temporary contract, or neither?

Decide to either commit to each other by getting married and being husband and wife, or make your final decision. It is unfair to be in limbo state like this. 

If after 3 years, you still do no know if you can share a picture or not, what have you both been doing? Only you can make the evaluation and judgment of how far to take this, and whether you should share your personal images, or not.

Please remember the purpose of relationships, especially from what our religion Islam says. 

And Allah knows best

Bismihi ta'āla

No, you cannot "renew" your nikah. What you can do is add conditions as a part of a binding contract ('aqd lazim), and it will be binding.

And Allah knows best