Practice these drill worksheets to master the basics of addition.
One-minute each day for two weeks will make a big difference!
Start from adding 1 and 2 (or even 3) each day. As 100% accuracy in 1 minute is mastered, proceed to 4,5,6, and so forth.
Basic math facts skill is extremely crucial in future math and I can tell you that with confidence as a secondary math educator.
Try my Timed Subtractions Facts
Try my Timed Multiplication Facts
More math resources also available @mathologist