How good are your side mirrors? If you don't have big ass mirrors with fish eyes you need to get some. I do a lot of driving vans/trucks and I always make sure my mirrors are spot on for visibility.
Also there is a certain element of tonnage wins. Set yourself up for the merge indicate your intent with your turn signal and then execute the merge. I don't want you to think you need to be dangerous when you drive. But it is not unreasonable to expect other drivers to give you room to merge. If you are feeling timid and don't make it clear that you will be merging they will not make room. But if you signal and make it clear that you will be needing a little space. Most drivers are good enough to understand that you are doing your best and just need a little room.
This is what I mean by big ass mirrors. You can see the fish eye mirror on the bottom. They will give you great visibility. If you don't have them you can get add on tow mirrors. Second picture