I need some advice, trolling motor inflatable raft.

Hi, I am looking for some advice. I have an intex excursion 5 inflatable raft. I just purchased a Newport 55lb thrust trolling motor for it, along with a group 27 marine battery. I have had the raft for 10 years but never use it becuase up until this weekend if we ever wanted to use it on a river it meant driving 2 cars. And leaving one at camp site and other at drop off location. Since we never want to drive 2 cars we always just rent a raft, or canoe. I don't like renting a raft when I have one, but I didn't have any better ideas. This weekend, I thought hay, why not buy a trolling motor and use it to get back up stream. This sort of worked, but not as good as I was hoping. Luckily I did not leave my car miles up stream and float down then try to get back upstream. We were on meramec river in Missouri. We were able to go a ways up stream but we hit a point 1/2 mile up were the current was just too strong. The little motor just did not have the power to push through the current. In places were the current was either not detectable, or just mild we moved really good, but in is place you could bearly stand up. The river was moving 5mph or more. I was really hoping this setup would work. while we had a good time, we were not able to go up stream as we hoped. I really did not want to change to a different trolling motor and battery set up if not needed. I might not have a choice though, If I want to be confident in getting up stream. I also don't want to buy something different without getting some advice on what would work. I picked the 55lb Newport after reading a lot online and people saying it would do what I needed. I thought about trying a 24v 86lb motor, but might have to eat the cost of mine, and fitting another big battery in the raft will be hard. I Am on a limited budget, and need to be able to pack it away and bring it with our popup camper and SUV. Any advice? Is there something I can do to this motor? Different prop? I don't know much about this stuff, but I can read and learn. Thanks