Sender Av:
a) Never ever ever give up or lose hope however many times you sin, Hashem is our father. I’m not sure if you are a parent who knows what unconditional love feels like, but Hashem has that in an infinite form.
b) Losing the sensitivity to an aveira after commiting it many times is human nature and is noted in Chazal (gemara Kiddushin: end of Perek 1). It does not nullify true tshuva.
c) Your tshuva is sincere as long as you honestly and deeply commit to mending your ways. No 2 ways about it. You are either committed or not. If you slip up after the commitment – that does happen but it should really be a complete surprise to even yourself if your commitment was honest. If you sort of saw that coming even while doing tshuva, it can’t be that sincere.