I thought I was hopeless at applied maths when I graduated from my undergrad. But after 3 years of graduate research, I still won't say I am good at it, but at least I can actually read papers in reasonable time now.
If you keep doing it, you will get better as you acquire more practice and experience. E.g. 1 year ago I was completely hopeless at cooking, but after stubbornly trying to cook over and over again, I can actually produce some tasty dishes. Thinking back, I wonder why I was ever stuck in these two activities. Same pattern happens for all my other hobbies.
In general, I have three broad advice:
Keep doing it and do it often. Being good at something is about doing it a lot over a long period of time.
Keep repeating the same thing. E.g. I used to be really bad at baking scones. Every time I failed, I would try to figure out why I failed and experiment with fixes. After 7 batches of sad looking squashed scones, I fixed all my mistakes and is now able to consistently bake nice looking scones!
3) Don't Delve on Specific Details
In my humble opinion, the biggest thing that has been holding me back from all my activities (mathematics or otherwise) was my inability to move ahead. I tend to get stuck trying to solve specific problems.
I find that it is much better to move on and make as much progress as possible, then come back to the stuck part later to try at it again. If I am unable to resolve it, I would move on again and then come back later. E.g. if you get stuck at a part of the paper, it might help to move on and read the rest of the paper. Or even put this paper aside and read another one.
A mathematics specific advice: it helps a great deal if you have supervision and/or feedback from a professor.