Best advice for someone feeling stuck, unmotivated, unable to make a decision or move forward?

My advice to someone who is feeling stuck, unmotivated, unable to make a decision or move forward is to jump in the deep end and do it. Without risk you don't get reward. Easier said than done absolutely acknowledged.

The words "All you can do is try your best with what you have" tends to be a good thing to say to yourself. when you are feeling this way. When feeling stuck it can also help to set and or have reasonable expectations about yourself and life. Sometimes we spend so much time trying to get to a point in life that maybe we set for ourselves, society sets for us, or maybe even our family or friends set for us. 

Given the information you explained, it sounds like you have been trying very hard with what you have and as you said you are scared.

There is a "What if" in what you are saying and it is absolutely justified.

You can go through and weigh the pro's and the con's, the good's and the bad's of a situation where you feel stuck, however a person will justify the answer that they want. In my experience I have found that it is helpful to jump in the deep end and or go for it. Then when you get scared or struggle utilize the tools you have. The tools that you have been using in the past during difficult times or when you have felt stuck. Reshape those tools to help motivate and push yourself through the tough sticky glue of the situation. 

I cannot tell you to stay or that decision must be yours. I live by the motto when you do positive things you get Positive results. So for you the translation may be taking the leap, making a move is a positive action even if it feels negative. 

The initial jump or leap and then the utilization of existing tools that may just need to be done differently has been an observed success for many individuals. When times get tough it can be helpful to look at the times it was tough in the past and think to yourself, how did I get through that? The way you got through it is a self-preservation factor and those factors can help to motivate and push you through lack of motivation leading to unhealthy habits. 

I hope this advice is helpful. Remind yourself jump in and utilize your tools. The tools just may need to be reshaped to fit the situation now.