It is entirely possible that your MP may not change their mind. But please do not let that hold you back from registering your concerns with your MP.
They may very well decide not to act on your wishes. But by sending them a message you make sure that they know many of their constituents have similar concerns and at the very least you make sure they cannot say that none of their constituents are worried about the issue. MPs talk to eachother, and even if a particular MP is unlikely to take the action we want, we still want them to be getting messages and to be discussing them with their colleagues.
Our campaign strategies take into account the fact that we know we may never convince every MP to back us. Some will already be on our side on a given issue; some will be persuadable to support our campaign; some will just be persuaded to not oppose or block our campaign; and some will disagree with us. But no matter where your MP falls on this spectrum, it is important that every MP hears about the campaign.