It would help if you posted a game with analysis. Looking at some of your games, youre not losing because of not knowing an opening.
Should you play the Modern Line against the Slav?

Yes thats a possible move. If that is what youre looking at to play, then i would suggest playing over the games of the following:
Aleksej Aleksandrov
Shakhriyar Mamedyarov
Vojislav Milanovic

The slav is freeing the Bc8 with 4...pxp 5a4 Bf5, and 4...e6, for some reason very popular with inexperienced players on chess com, the semi-slav but at least know what talking about now.
I just wanted to say that I played the triangle slav against someone today and I saw something weird. After I pinned the knight with Bb4 I was expecting a bd7 move to break the pin in the first 10 moves but instead I saw nd7. A couple moves later when he took back he brought the bishop out on the diagonal and I couldn't castle. I'm not sure how the game went but I think it was something along the lines of this,
1. Nf3 d5
2, c4 c6
3. d4 e6
4. nc3 dxc4
5, a4 Bb4
6. e4 b5
7. axb5 cxb5
8. nd2 Bxc3
9. bxc3 nf6
10. ba3
And I fell under pressure, Not sure that was the order because I think there was a c pawn less advances but there was some off nd2 that I didn't understand at first.
I am having trouble beating the Slav, do you guys have any suggestions?