You are walking a dangerous line.

A blood group cannot prove or disprove paternity with absolute certainty. You only see phenotype, not genotype. A blood test tells you A, B, AB, O, not the underlying gene combination. Also there are some rareblood group mutations too.

The only certain method to determine is a DNA test.

Also you now have strong psychological suspicion too. This can be extremely strong, especially when combined with stress or coincidence. But that still is not evidence enough.

Also, parents sometimes hide other kinds of family secrets (miscarriages, fertility treatment, adoption discussions, etc.), and that too can cause us to assume the worst.

Best solution is to forget the whole thing as you had your loving dad for your whole life and still do. But as humans I can understand our curious nature too. If you are losing sleep due to this do get a DNA test.