Hi, I am sorry about what you are going through and I understand how frustrating it can be with knowing what to do from here. I believe it is very difficult to go by what it says online because it really just will give you general advice that may not pertain to your individual situation. However, based on the information that you gave me I think it would be good to let him know that you would like to have a serious talk with him about how the relationship is going. During the talk everything should be discussed in a calm manner without arguing. I would express to him the concerns you are having and go from there. I would also ask him questions as to why he gets upset over minor issues and how he sees the relationship going. By asking him these questions it can help you to understand how he thinks and feels about the relationship and also may help him to stop and realize how he is behaving. However, based on what you explained it seems to me that he may be unwilling to talk about this. If this is true you can try writing him a letter explaining the difficulties you are having in the relationship and how you need to be able to speak to him in order for the relationship to move forward. A letter is good because it will give you the chance to perfect exactly what you want to say and a chance for him to dwell on what you wrote rather than respond in hasty fashion. I understand you do not want to back off and you want to mke this work especially after you put your all into it. This is why I gave you those options above. All you can do is your part. Unfortunately if he is still unwilling to do is then it may be best to move on. It would not be because you need to move on, but it may be the best decision in your best interest. I say this because I would hate for you to lose any more of your precious time. He may be seeing someone or maybe not there is really no way to know unless he honest about it or if you see signs such as his actions not matching what he says and etc. Being long distance makes it all the more harder to know. However, whatever the reason he is behaving like this is not good especially since there has been trust issues in the past. If he is unwilling to do his part, prove his trust to you, treat you good and with respect it may not be the best for you. I understand backing off is difficult, but you owe to yourself to have someone you can trust and that will give you what you deserve rather than how he is treating you. It is not fair to you that you are afraid to call him and that he does not answer. A healthy relationship is based on love and he should be welcoming you and this love should shine through his voice and actions. I know you are worried about the fact you already put a lot of time into it, but I would not want you do not find yourself in the same situation 5 years from now. But like I said if you want to save the relationship try discussing first. If he feels the same as you do then he should be able to converse and clear miscommunication in order to strengthen the relationship and make things better moving forward.
I wish you well and please do not hesitate if I can be of further help to you.