tips on strengthening shoulders?

Hey guys,
So in the past month I've gained a couple pounds, but am still less than ten lbs away from my goal weight. However, i am trying to add strength training/weights into my routine.. only problem is that I have bad shoulders from years of competitive swimming. Even doing planks or push ups kill my shoulders, but though the rest of my body is pretty strong. I don't have sharp or shooting pains in my shoulders, just a dull, weak feeling.
Are there any exercises that will build up my shoulders and decrease the pain or am I stuck with bad shoulders the rest of my life? I was hoping that my shoulders would gradually get stronger by working through the discomfort and it just doesn't seem to be working. Any help??
Thanks! Lindsey

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